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: if 5 a = 3125, then the value of 5 ( a - 3 ) is :
Options: a. 25, b. 125, c. 625, d. 1625, e. none of these
Example Output: a

Example Input: Problem: the present population of a town is 2000. population increase rate is 20 % p. a. find the population of town after 1 years ?
Options: a. 1000, b. 2400, c. 1500, d. 1600, e. 1250
Example Output: b

Example 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
Example Output:
b