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 an investor puts $ 800 in a savings account that earns 10 percent annual interest compounded semiannually, how much money will be in the account after one year ?
Options: a. $ 878, b. $ 880, c. $ 882, d. $ 884, e. $ 886
Example Output: c

Example Input: Problem: a sum of money at simple interest amounts to rs. 720 after 2 years and to rs. 1020 after a further period of 5 years. the sum is :
Options: a. 400, b. 500, c. 600, d. 700, e. 800
Example Output: c

Example Input: Problem: if 4 / ( 1 + 3 / x ) = 1, then x =
Options: a. 3, b. 1, c. 1 / 3, d. - 1 / 3, e. - 3
Example Output:
b