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.

Ex Input:
Problem: a rope of which a calf is tied is increased from 12 m to 23 m, how much additional grassy ground shall it graze ?
Options: a. 2989, b. 2789, c. 1210, d. 1288, e. 1211

Ex Output:
c


Ex Input:
Problem: there are 7 pairs of socks and 2 socks are worn from that such that the pair of socks worn are not of the same pair. what is the number of pair that can be formed.
Options: a. 40, b. 49, c. 41, d. 56, e. 42

Ex Output:
e


Ex Input:
Problem: the least perfect square, which is divisible by each of 21,36 and 66 is
Options: ['a. 213444', 'b. 214344', 'c. 214434', 'd. 213443', 'e. none of these']

Ex Output:

a