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: an engine moves at the speed of 30 kmph without any coaches attached to it. speed of the train reduces at the rate that varies directly as the square root of the number of coaches attached. when 9 coaches are attached speed decreases to 18 kmph. what will be the speed of train when 16 coaches are attached.
Options: a. 14, b. 16, c. 18, d. 20, e. 22

Ex Output:
a


Ex Input:
Problem: what is the perimeter of a square field whose diagonal is 8 â ˆ š 2 ?
Options: a. 64 m, b. 32 m, c. 30 m, d. 16 m, e. 18 m

Ex Output:
b


Ex Input:
Problem: in a group of 90 students, 30 are taking history, and 32 are taking statistics. if 59 students are taking history or statistics or both, then how many students are taking history but not statistics ?
Options: a. 9, b. 19, c. 33, d. 27, e. 31

Ex Output:

c