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 two dice are thrown together, the probability of getting an even prime number on the dice is
Options: a. 1 / 36, b. 1 / 18, c. 1 / 13, d. 1 / 9, e. 5 / 36
Example Output: a

Example Input: Problem: how many positive even integers less than 100 contain digits 5 or 7 ?
Options: a. 10, b. 17, c. 18, d. 19, e. 20
Example Output: a

Example Input: Problem: vishal invested 10 % more than trishul. trishul invested 10 % less than raghu. if the total sum of their investments is rs. 5780, how much amount did raghu invest ?
Options: a. 1287, b. 2887, c. 2000, d. 1129, e. 1192
Example Output:
c