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: what is the units digit of 32 ! + 50 ! + 4 ! + 4 ! ?
Options: a. 0, b. 2, c. 4, d. 6, e. 8
Example Output: e

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: if - 3 x + 5 y = 48 and 3 x - 2 y = 6, what is the product of x and y ?
Options: a. 252, b. 428, c. 464., d. 200, e. 642
Example Output:
a