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: if n = 2 ^ 0.15 and n ^ b = 1, b must equal
Options: a. 3 / 80, b. 20 / 3, c. 4, d. 5 / 3, e. 80 / 3

Ex Output:
b


Ex Input:
Problem: if s is the sum of reciprocals of a list of consecutive integers from 55 to 64, inclusive, s is approximately equal to
Options: a. 0.16, b. 0.2, c. 0.3, d. 0.4, e. 0.5

Ex Output:
a


Ex Input:
Problem: two trains, one from howrah to patna and the other from patna to howrah, start simultaneously. after they meet, the trains reach their destinations after 25 hours and 16 hours respectively. the ratio of their speeds is ?
Options: a. 4: 6, b. 4: 3, c. 4: 9, d. 4: 5, e. 4: 2

Ex Output:

d