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: in 10 years, a will be twice as old as b was 10 years ago. if a is now 5 years older than b the present age of b is
Options: a. 37, b. 38, c. 39, d. 35, e. 41
Example Output: d

Example Input: Problem: the total age of a and b is 16 years more than the total age of b and c. c is how many year younger than
Options: a. 14 years, b. 12 years, c. 56 years, d. 16 years, e. 55 years
Example Output: d

Example Input: Problem: the compound interest earned by sunil on a certain amount at the end of two years at the rate of 10 % p. a. was rs. 420. find the total amount that sunil got back at the end of two years in the form of principal plus interest earned.
Options: a. rs. 2530, b. rs. 2430, c. rs. 2420, d. rs. 2410, e. rs. 2440
Example Output:
c