Given a math problem with context and a question and 5 answer choices, the task is to provide the correct answer choice based on the problem. You must choose one of the given answer choices by letter: a, b, c, d, or e; anything else is invalid.

[Q]: Problem: a merchant marks his goods up by 30 % and then offers a discount of 10 % on the marked price. what % profit does the merchant make after the discount ?
Options: a. 17 %, b. 25 %, c. 69 %, d. 31 %, e. 19 %
[A]: a


[Q]: Problem: if the sides of a cube are in the ratio 5: 3. what is the ratio of their diagonals ?
Options: ['a. 5: 9', 'b. 9: 3', 'c. 5: 4', 'd. 3: 5', 'e. 5: 3']
[A]: e


[Q]: Problem: x can finish a work in 21 days. y can finish the same work in 15 days. yworked for 10 days and left the job. how many days does x alone need to finish the remaining work ?
Options: a. 3, b. 5, c. 7, d. 8, e. 9
[A]:
c