OS Quiz Questions and answers

.README.md

Contributing

1. Fork the gist with the button in the top right corner
2. Make your changes (able to do so in browser, using the edit tool)
3. Add a bullet point to changes.md with a note about what you're adding
4. Make a comment on this main gist with a link to your gist.

As soon as I can, I will merge your changes into this main gist!

Understanding the file format

Files are formated using markdown. See here.

Question formating

Each file contains the questions and answers for one quiz. Unanswered questions are bolded. Questions have the following format:

- What is the color of the sky?
  - Blue

Don't include wrong answers, except for choose all that apply, in that case, incorrect and correct answers should be listed, with [ ] for incorrect answers and [X] for correct ones. The question should be followed by (Check all). Example:

- Which classes at UF make me want to die? (Check all)
  - - [X] OS
    - [ ] All other classes
    - [ ] Any other class

For questions with blanks, use _____ to denote blanks. Example:

- In Linux and Unix, the type of disk is determined by the `_____` device number, while the particular volume is identified by the `_____` device number.
  - major, minor

For questions with matching, make a list of answers, with their matches being in italics. Example:

- What type of deadlock handling method is each of the following?
  - Banker's Algorithm: *Deadlock avoidance*
  - Total ordering of resource types: *Deadlock prevention*
  - Preemption, rollback, or killing blocked process(es): *Deadlock detection and recovery*
  - Full speed ahead an hope for the best: *Ostrich algorithm*

Adding Explanations

To avoid getting the files too cluttered, add you explanation as a collabsable section after the answer for the question you want to explain. Example:

- What is the color of the sky?
  - Blue
  <details>
    <summary>6XAM's Explanation</summary>
    This is due to the properties of light. Checkout this wikepedia article: https://en.wikipedia.org/wiki/Diffuse_sky_radiation
  </details>

This will display as below:

• What is the color of the sky?
  • Blue
  6XAM's Explanation

  This is due to the properties of light. Checkout this wikepedia article: https://en.wikipedia.org/wiki/Diffuse_sky_radiation

09 Files Quiz.md

Quiz 9 Files

• When is the reference count in an i-node increased? (Check all)

  • • When a process with the file open forks
    • When a process writes to the file
    • When a process reads from the file
    • When a directory entry is linked to the file
  6XAM's Explanation

  The reference count for an i-node is the number of other directory entries that refer to this specific file (aka this specific i-node). None of the other choices create a new entry refering to this file, so they are not correct. Another example of something that would decrease an i-node's reference count is deleting a directory entry.

• Which of the following types of file is most likely to exhibit strong locality of access? (Check all)

  • • Database
    • Digital Movie
    • Audio File
    • Electronic Novel
    • Spanish-English Dictionary
  6XAM's Explanation

  Locality of access refers to the idea that accesses close to each other in time should be close to each other in location on the disk. The Database and Dictionary have fairly random access, so there is no strong locality of access. With all of the other data, there is some sequence to the data that is generallay followed. Even though you may skip around a video, song, or book, you generally will watch/listen/read in order.

• What are strong arguments for having larger file block size? (Check all)

  • • Desired data are more likely to be in the block cache for localized accesses.
    • Larger blocks are less expensive.
    • The data rate for reading a block into memory is higher.
    • It takes less time to load a block.
  6XAM's Explanation

  Larger file blocks mean that you're file will be chopped up into fewer, larger chunks.
  

  1. This means local accesses, which are going to be close to each other, will likely be in the last-read (chached) block.
     
  2. Larger blocks are more expensive than small ones because you have internal fragmentation (up until you get to super small block sizes, in which case you have to start considering the size of the data structure that stores the list of blocks).
     
  3. The data rate is higher because you can read in a whole block before having to spend reading the FCB
     
  4. Larger blocks obviously take more time to load than smaller blocks.

• How many disk accesses are needed to bring byte i of a file into memory when the file is stored using contiguous allocation? Assume only the file's FCB is in memory, block pointers require 32 bits, and that blocks hold 4096 bytes each.

  • 1 access
  6XAM's Explanation

  Because the FCB is in memory, we don't need to access the disk to read it. In contiguous allocation, the file is stored directly at it's location on the disk in one long segment, so to get byte i of it, we just need to add i to the file's location from the FCB, and can read that byte in one disk access.

• How many disk accesses are needed to bring byte i of a file into memory when the file is stored using double indirect indexed allocation? Assume only the file's FCB is in memory, block pointers require 32 bits, and that blocks hold 4096 bytes each.

  • 3 accesses
  6XAM's Explanation

  Because the FCB is in memory, we don't need to access the disk to read it. In double indirect indexed allocation, the FCB contains a list-of-lists-of-lists of blocks that the file is stored in. So, we need to read the block containing the first list, then from that list we need to read the block containing the second list, and then finally we can read the block that contains our data.

• How many disk accesses are needed to bring byte i of a file into memory when the file is stored using indexed allocation? Assume only the file's FCB is in memory, block pointers require 32 bits, and that blocks hold 4096 bytes each.

  • 2 accesses
  6XAM's Explanation

  Because the FCB is in memory, we don't need to access the disk to read it. In double indirect indexed allocation, the FCB contains a list-of-lists of blocks that the file is stored in. So, we need to read the block containing the list, and then we can read the block that contains our data.

• What is the largest file size that can be allocated if blocks hold 4096 bytes, block pointers are 32 bits, and files are stored using indexed allocation?

  • 4MB
  6XAM's Explanation

  Block pointers are 32 bits, and with indexed allocation, we have 1 block where we store a list of the blocks that this file is made up of. So, that block of 4096 bytes can hold (4096 bytes/32 bits) block pointers. Unit conversions gives us ((4096*8) bits/32 bits) = 1024 block pointers, meaning a file can be made of, at maximum, 1024 blocks, or 1024*4096 bytes = 4194304 bytes. More unit conversion gives 4194304 bytes * (1 mb / 1024^2 bytes) = 4 mb

• What is the largest file size that can be allocated if blocks hold 4096 bytes, block pointers are 32 bits, and files are stored using double indexed allocation?

  • 4GB
  6XAM's Explanation

  Block pointers are 32 bits, and with indexed allocation, we have 1 block where we store a list of the blocks that store the actual pointers to the blocks this file is made up of. So, one block of 4096 bytes can hold (4096 bytes/32 bits) block pointers. Unit conversions gives us ((4096*8) bits/32 bits) = 1024 block pointers. In this case, we have one block that could be potentially full of pointers to blocks that themselves contain the block pointers for this file, so we have 1024 blocks each holding 1024 block pointers, or 1024 * 1024 block pointers, meaning a file can be made of, at maximum, 1024 * 1024 blocks, or 1024 * 1024 * 4096 bytes = 4294967296 bytes. More unit conversion gives 4294967296 bytes * (1 gb / 1024^3 bytes) = 4 gb

• What is the largest file size that can be allocated if blocks hold 4096 bytes, block pointers are 32 bits, and files are stored using linked allocation?

  • 16TB
  6XAM's Explanation

  Block pointers are 32 bits, and with linked allocation the files could theoretically span all the blocks of the system (the linked list for one file goes thru all the blocks). So, to figure out the maximum number of blocks we can have, we simply need to figure out how many unique block pointers we can have; in this case, the pointer is 32 bits, so we have up to 2^32 blocks per file. Multiplying this by the size of the blocks gives us our file size, 2^32 * 4096 bytes = 16TB.

• What is the largest file size that can be allocated if blocks hold 4096 bytes, block pointers are 32 bits, and files are stored using contiguous allocation?

  • 16TB
  6XAM's Explanation

  Block pointers are 32 bits, and with contiguous allocation the files could theoretically span all the blocks of the system (Because with contiguous allocation, the only limit is the hard drive size). So, to figure out the maximum number of blocks we can have, we simply need to figure out how many unique block pointers we can have; in this case, the pointer is 32 bits, so we have up to 2^32 blocks per file. Multiplying this by the size of the blocks gives us our file size, 2^32 * 4096 bytes = 16TB.

10 Disk Quiz.md

Quiz 10 Disk

• A disk takes 1ms per track seek time, and the head is currently on track 70. Assume it takes 5ms to service a request once the head is at the right track. The following set of requests are pending, given as (ID, track) in order of arrival. How much time does it take to service all the requests if the elevator algorithm is used an the current direction is down? Requests: (A,80)(B,75)(C,82)(D,90)(E,45)

  • 95 ms
  TylerMetzger's Explanation

  The elevator algorithm (a.k.a. SCAN algorithm) moves in a given direction, servicing the requests in its path. Once reaching the final request in that direction, it reverses its direction and follows the same pattern.

    Next Served       Distance to Travel        Current Direction
      E - 45                 25                       Down
      B - 75                 30                         Up
      A - 80                  5                         Up
      C - 82                  2                         Up
      D - 90                  8                        N/A
    

  To calculate the total time add the travel distance times the seek time per track plus the total tracks serviced times service time per track:

    (25+30+5+2+8) tracks * 1 ms/track + 5 tracks * 5ms/track = 70ms (seek time) + 25ms (service time) = 95ms
    

• A disk takes 1ms per track seek time, and the head is currently on track 50. Assume it takes 5ms to service a request once the head is at the right track. The following set of requests are pending, given as (ID, track) in order of arrival. What is the order in which requests are serviced if the policy is SSF (aka SSTF)? Requests: (A,55)(B,53)(C,40)(D,48)(E,45)

  • DECBA
  TylerMetzger's Explanation

  The SSTF algorithm (Shortest Seek Time First) prioritizes the shortest distance between the current track and a pending request.

    Current Track       Distance to Travel        Shortest Distance
          50           A-5  B-3 C-10 D-2 E-5             D-2
      D - 48           A-7  B-5 C-8      E-3             E-3
      E - 45           A-10 B-8 C-5                      C-5
      C - 40           A-15 B-13                         B-13
      B - 53           A-2                               A-2
      A - 55                    N/A                      N/A
    

• A disk takes 1ms per track seek time, and the head is currently on track 50. Assume it takes 5ms to service a request once the head is at the right track. The following set of requests are pending, given as (ID, track) in order of arrival. How much time does it take to service all the requests if the policy is SSF (aka SSTF)? Requests: (A,55)(B,53)(C,40)(D,48)(E,45)

  • 50ms
    TylerMetzger's Explanation

    The SSTF algorithm (Shortest Seek Time First) prioritizes the shortest distance between the current track and a pending request.

    Current Track       Distance to Travel        Shortest Distance
          50           A-5  B-3 C-10 D-2 E-5             D-2
      D - 48           A-7  B-5 C-8      E-3             E-3
      E - 45           A-10 B-8 C-5                      C-5
      C - 40           A-15 B-13                         B-13
      B - 53           A-2                               A-2
      A - 55                    N/A                      N/A 
    

    To calculate the total time add the travel distance times the seek time per track plus the total tracks serviced times service time per track:

    (2+3+5+13+2) tracks * 1 ms/track + 5 tracks * 5ms/track = 25ms (seek time) + 25ms (service time) = 50ms
    

• A disk takes 1ms per track seek time, and the head is currently on track 70. Assume it takes 5ms to service a request once the head is at the right track. The following set of requests are pending, given as (ID, track) in order of arrival. What is the order in which the requests are serviced if the Elevator algorithm is used and the current direction is down? Requests: (A,80)(B,75)(C,82)(D,90)(E,45)

  • EBACD
  TylerMetzger's Explanation

  The elevator algorithm (a.k.a. SCAN algorithm) moves in a given direction, servicing the requests in its path. Once reaching the final request in that direction, it reverses its direction and follows the same pattern.

  Next Served       Distance to Travel        Current Direction
    E - 45                 25                       Down
    B - 75                 30                         Up
    A - 80                  5                         Up
    C - 82                  2                         Up
    D - 90                  8                        N/A
  

• A disk takes 1ms per track seek time, and the head is currently on track 50. Assume it takes 5ms to service a request once the head is at the right track. The following set of requests are pending, given as (ID, arrival time in ms, track). When is the last request finished if the policy is SSF (aka SSTF)? Requests: (A,3,55)(B,4,53)(C,8,40)(D,10,48)(E,32,45)

  • 53 ms
  TylerMetzger's Explanation

  The SSTF algorithm (Shortest Seek Time First) prioritizes the shortest distance between the current track and a pending request. The SSTF algorithm is non-preemptive, that is, if a new request appears that is closers to the current track then the current request being seeked, it will not interrupt the current seek, and will have to wait to be serviced after the current request.

    Time Quantumn      Current Action      Current Track       Distance to Travel        Shortest Distance
        0-3               Waiting                50           A-5                               A-5
        3-8            Seek Request A         50-55                   N/A                       N/A
        8-13             Service A               55               B-2 C-15 D-7                  B-2
       13-15           Seek Request B         55-53                   N/A                       N/A
       15-20             Service B               53                   C-13 D-5                  D-5
       20-25           Seek Request D         53-48                   N/A                       N/A
       25-30             Service D               48                   C-8                       C-8
       30-38           Seek Request C         48-40                   N/A                       N/A
       38-43             Service C               40                            E-5              E-5
       43-48           Seek Request E         40-45                   N/A                       N/A
       48-53             Service E               45                   None                      None
     

• A disk takes 1ms per track seek time, and the head is currently on track 50. Assume it takes 5ms to service a request once the head is at the right track. The following set of requests are pending, given as (ID, arrival time in ms, track). What is the order in which the requests are serviced if the policy is SSF (aka SSTF)? Requests: (A,3,55)(B,4,53)(C,8,40)(D,10,48)(E,32,45)

  • ABDCE
  TylerMetzger's Explanation

  The SSTF algorithm (Shortest Seek Time First) prioritizes the shortest distance between the current track and a pending request. The SSTF algorithm is non-preemptive, that is, if a new request appears that is closers to the current track then the current request being seeked, it will not interrupt the current seek, and will have to wait to be serviced after the current request.

    Time Quantumn      Current Action      Current Track       Distance to Travel        Shortest Distance
        0-3               Waiting                50           A-5                               A-5
        3-8            Seek Request A         50-55                   N/A                       N/A
        8-13             Service A               55               B-2 C-15 D-7                  B-2
       13-15           Seek Request B         55-53                   N/A                       N/A
       15-20             Service B               53                   C-13 D-5                  D-5
       20-25           Seek Request D         53-48                   N/A                       N/A
       25-30             Service D               48                   C-8                       C-8
       30-38           Seek Request C         48-40                   N/A                       N/A
       38-43             Service C               40                            E-5              E-5
       43-48           Seek Request E         40-45                   N/A                       N/A
       48-53             Service E               45                   None                      None
     

• A disk takes 1ms per track seek time, and the head is currently on track 70. Assume it takes 5ms to service a request once the head is at the right track. The following set of requests are pending, given as (ID, arrival time in ms, track). What is the order in which the requests are serviced if the Elevator algorithm is used an the current direction is down? Requests: (A,0,80)(B,4,75)(C,12,82)(D,20,90)(E,32,45)

  • ACDBE
  TylerMetzger's Explanation

  The elevator algorithm (a.k.a. SCAN algorithm) moves in a given direction, servicing the requests in its path. Once reaching the final request in that direction, it reverses its direction and follows the same pattern. The elevator algorithm is non-preemptive, that is, if a new request appears that is closers to the current track then the current request being seeked, it will not interrupt the current seek, and will have to wait to be serviced after the current request.

    Time Quantumn      Current Action      Next Served       Distance to Travel        Current Direction
           0                Wait              A-80                   10                       Down 
        0-10           Seek Request A         N/A                    N/A                       Up
       10-15             Service A            C-82                    2                        Up
       15-17           Seek Request C         N/A                    N/A                       Up
       17-22             Service C            D-90                    8                        Up
       22-30           Seek Request D         N/A                    N/A                       Up
       30-35             Service D            B-75                   15                       Down
       35-50           Seek Request B         N/A                    N/A                      Down
       50-55             Service B            E-45                   30                       Down
       55-85           Seek Request E         N/A                    N/A                      Down
       85-90             Service E            None                   None                     Down
    

• A disk takes 1ms per track seek time, and the head is currently on track 70. Assume it takes 5 ms to service a request once the head is at the right track. The following stream of requests arrive, given as (ID, arrival time in ms, track). When is the last request finished if the Elevator algorithm is used and the current direction is down? Requests: (A,0,80)(B,4,75)(C,12,82)(D,20,90)(E,32,45)

  • 90 ms
  nihamaity's Explanation

  To A from current: 80-70 = 10, plus 5 ms to service request: 10 + 5 = 15 ms. To C from A: 82-80 = 2, plus 5 ms to service request: 2 + 5 = 7 ms. To D from C: 90-82 = 8, plus 5 ms to service request: 8 + 5 = 13 ms. To B from D: 90-75 = 15, plus 5 ms to service request: 15 + 5 = 20 ms. To E from B: 75-45 = 30, plus 5 ms to service request: 30 + 5 = 35 ms. Add all times to get the time of when the last request is finished: 15 + 7 + 13 + 20 + 35 = 90 ms.

11 IO Quiz.md

Quiz 11 IO

• Suppose you have a harddrive that spins at 9000 RPM, and has 80 sectors of 512 bytes each on a particular track. If the head is already in position when a request for a single sector of that track is serviced, what is the expected effective data rate to get the data into the controller's buffer? Give the answer in BPS with 3 significant digits.

  • 150,000

• Suppose you have a hard drive that spins at 9000 RPM, and has 100 sectors of 512 bytes each on a particular track. If the head is already in position when a request for two adjacent sectors of that track is serviced, what is the expected effective data rate to get the data into the controller's buffer? Give the answer in Bps, with 3 significant digits.

• 301, 000

• The IBM 1402 Card Reader could read 800 cards per minute. Each card had 80 columns, each of which could hold one byte in EBCDIC. What was the approximate data rate of this machine in bytes/second (round to the nearest integer)

  • 1,067

• A typical US Keyboard has a total of 104 keys. If CTL, ALT, ALT2, and SHIFT may be used in any combination to modify any of the remaining keys, how many character codes can be generated, asssuming there is no distinction between left and right bersions of these four modifier keys (they all habe two versions) but there is a distinction between all other keys (e.g., the key labeled 1 above the Q key and the 1 key on the numeric keypad are not different)?

  • 1,536

• In Linux and Unix, the type of disk is determined by the _____ device number, while the particular volume is identified by the _____ device number.

  • major, minor

• The _____ codes passed from the keyboard to the driver are converted to character codes using a _____.

  • scan
  • keymap

• The IBM nine-track tapes that became the industry standard for storage for three decades had several sizes , the most common being 2400 feet. The high density version stored data at 1600 bits/inch/track. If the whole tape (not including the parity track) could be used for data storage , how much raw data could one of these tapes hold?(Round to the nearest MB using decimal MB not binary MB)

  • 46

12 Deadlock Quiz.md

Quiz 12 Deadlock

• In the following resource allocation scenario, which process(es) can be marked in the third iteration of the loop (but not the first or second iteration) in the Safe State Checking algorithm? (Check all)
  Exist = (5,4,5,3,2)
  Max:

  Proc	Res 1	Res 2	Res 3	Res 4	Res 5
  A	2	3	2	1	0
  B	2	1	1	0	1
  C	1	2	2	1	1
  D	2	1	1	1	1
  E	1	3	3	2	1

  Has:

  Proc	Res 1	Res 2	Res 3	Res 4	Res 5
  A	1	0	1	0	0
  B	1	1	0	0	1
  C	1	2	0	1	0
  D	0	1	1	0	1
  E	1	0	1	2	0

  • • A
    • B
    • C
    • D
    • E

• Consider the following scenario in a system with 5 resource types. (Check all)
  Exist = (5,4,5,3,2)
  Max:

  Proc	Res 1	Res 2	Res 3	Res 4	Res 5
  A	2	3	2	1	0
  B	2	1	1	0	1
  C	1	2	2	1	1
  D	2	1	1	1	1
  E	1	3	3	2	1

  Has:

  Proc	Res 1	Res 2	Res 3	Res 4	Res 5
  A	1	0	1	0	0
  B	1	1	0	0	1
  C	1	2	0	1	0
  D	0	1	1	0	1
  E	1	0	1	2	0

  • • A
    • B
    • C
    • D
    • E

• The following system state is a _____ state.
  Exist = (5,4,5,3,2)
  Max:

  Proc	Res 1	Res 2	Res 3	Res 4	Res 5
  A	2	3	2	1	0
  B	2	1	1	0	1
  C	1	2	2	1	1
  D	2	1	1	1	1
  E	1	3	3	2	1

  Has:

  Proc	Res 1	Res 2	Res 3	Res 4	Res 5
  A	1	0	1	0	0
  B	1	1	0	0	1
  C	1	2	0	1	0
  D	0	1	1	0	1
  E	1	0	1	2	0

  • safe

• In the following resource allocation scenario, which process(es) can be marked the second iteration through the loop (but not the first iteration) in the Safe State Checking algorithm? (Check all) Exist = (5,4,5,3,2)
  Max:

  Proc	Res 1	Res 2	Res 3	Res 4	Res 5
  A	2	3	2	1	0
  B	2	1	1	0	1
  C	1	2	2	1	1
  D	2	1	1	1	1
  E	1	3	3	2	1

  Has:

  Proc	Res 1	Res 2	Res 3	Res 4	Res 5
  A	1	0	1	0	0
  B	1	1	0	0	1
  C	1	2	0	1	0
  D	0	1	1	0	1
  E	1	0	1	2	0

  • • A
    • B
    • C
    • D
    • E

• What type of deadlock handling method is each of the following?

  • Banker's Algorithm: Deadlock avoidance
  • Total ordering of resource types: Deadlock prevention
  • Preemption, rollback, or killing blocked process(es): Deadlock detection and recovery
  • Full speed ahead an hope for the best: Ostrich algorithm

13 Security Quiz.md

Quiz 13 Security

• If a process in a MAC system has label <Secret, {Homeland Security}> files with which labels is it able to read, assuming the usual label meanings? (Check all)

  • • <Top Secret, {Homeland Security}>
    • <Confidential, {Homeland Security, Submarines}>
    • <Confidential, {}>
    • <Secret, {Homeland Security}>

• Which of the following are strengths of capability lists? (Check all)

  • • They make it easy to determine which domains can access an object and how.
    • They make it easy to revoke access rights to an object.
    • They make it easy to determine which objects a domain can access and how.
    • They store the ACM efficiently.

• Which of the following are strengths of ACLs?

  • • They make it easy to determine which domains can access an object and how.
    • They make it easy to revoke access rights to an object.
    • They make it easy to determine which objects a domain can access and how.
    • They store the ACM efficiently.

• What causes the greatest amount of loss to data?

  • User Errors

• What is the primary drawback of the Access Control Matrix as an implementation?

  • It is too large

• In what type of access control system are labels on objects and subjects used determine who can access it and how?

  • MAC

• In what type of access control system does the owner of an object determine who can access it and how?

  • DAC

14 Security2 Quiz.md

Quiz 14 Security2

• With a good symmetric cyptosystem (Check all)

  • • given only ciphertext C=E(K,P) it is difficult to obtain the corresponding plaintext P
    • given only ciphertext C=E(K,P) it is difficult to obtain the key K
    • given only plaintext P it is difficult to obtain the ciphertext C=E(K,P)
    • given the encryption key K it is difficult to obtain the plaintext P from ciphertext C=E(K,P)

• With a good asymmetric cryptosystem used in the normal way, for a member of the public (Check all)

  • • given ciphertext C it is difficult to obtain the corresponding plaintext P
    • given plaintext P it is difficult to obtain the ciphertext C
    • given the encryption key K it is difficult to obtain the plaintext P from ciphertext C
    • given only ciphertext C it is difficult to obtain the decryption key K'

• If you are using biometric authentication to restrict access to extremely dangerous pathogens in a biomedical research facility, what modification to the default settings of the authentication system would be wise?

  • Modify the acceptance threshold to make the FRR as near to zero as possible

• The key difference between a virus and a worm is:

  • a virus must be attached to another file

• Which of the following are drawbacks to password-based authentication? (Check all)

  • • Not all users may be able to enroll.
    • Passwords may change with physical condition.
    • Passwords may be misplaced or lost
    • Passwords may be forgotten.
    • Passwords may be guessed.
    • Passwords may be observed.
    • Users may be reluctant to use a password.
    • Passwords may be used for purposes other than authentication.

• Which of the following are drawbacks to biometric authentication (under normal circumstances)? (Check all)

  • • Not all users may be able to enroll.
    • Biometrics may change with physical condition.
    • Biometric keys may be lost.
    • Biometrics may be forgotten.
    • Users may not want to submit to biometric evaluation.
    • Biometrics may be used for purposes other than authentication.

changes.md

Contribution log

• Initialized Repo
• Initial Contributions from LukaAntoljak, nihamaity, and qingyuan.1016
• Quiz 9 explanations adde by 6XAM
• Quiz 14 added by qingyuan.1016
• Quiz 10 explanations added by TylerMetzger
• Quiz 11 question added by qingyuan.1016
• Quiz 11 added a question by sudeep
• Quiz 10 question added by nihamaity