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 a person walks at 25 km / hr instead of 10 km / hr, he would have walked 20 km more. the actual distance traveled by him is :
Options: a. 50 km, b. 56 km, c. 13.3 km, d. 70 km, e. 80 km
Example Output: c

Example Input: Problem: find the simple interest on rs. 72,000 at 16 2 / 3 % per year for 9 months.
Options: a. 7500, b. 6500, c. 9000, d. 9500, e. none of them
Example Output: c

Example Input: Problem: in an election between two candidates, one got 60 % of the total valid votes, 30 % of the votes were invalid. if the total number of votes was 9000, the number of valid votes that the other candidate got, was :
Options: a. 2500, b. 2520, c. 3000, d. 3100, e. none of these
Example Output:
b