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.

Example Input: Problem: the difference between the compound interest compounded annually and simple interest for 2 years at 20 % per annum is rs. 216. find the principal ?
Options: a. 2277, b. 2667, c. 5400, d. 2766, e. 1811
Example Output: c

Example Input: Problem: what is the least number of squares tiles required to pave the floor of a room 15.17 m long and 9.02 m broad ?
Options: ['a. 814', 'b. 266', 'c. 287', 'd. 999', 'e. 661']
Example Output: a

Example Input: Problem: a does half as much work as b in 3 - fourth of the time. if together they take 18 days to complete the work, how much time shall b take to do it ?
Options: a. 30 days, b. 35 days, c. 40 days, d. 45 days, e. 50 days
Example Output:
a