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: what will be the compound interest on a sum of rs. 20,000 after 3 years at the rate of 12 % p. a. ?
Options: a. rs. 8098.56, b. rs. 9720, c. rs. 10123.20, d. rs. 10483.20, e. none

Ex Output:
a

Ex Input:
Problem: a man can row upstream at 25 kmph and downstream at 55 kmph, and then find the speed of the man in still water ?
Options: a. 65, b. 40, c. 30, d. 78, e. 38

Ex Output:
b

Ex Input:
Problem: in an election, candidate douglas won 58 percent of the total vote in counties x and y. he won 64 percent of the vote in county x. if the ratio of people who voted in county x to county y is 2: 1, what percent of the vote did candidate douglas win in county y ?
Options: a. 15 %, b. 18 %, c. 25 %, d. 36 %, e. 46 %

Ex Output:
e