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: i flew my tiny seaplane to visit my mother. on the flight up, i flew at 96 mph. on the way home, i flew 88 mph. what was my average speed for the trip ?
Options: a. 198 mph, b. 110 mph, c. 92 mph, d. 100 mph, e. 99 mph
Example Output: c

Example Input: Problem: if a - b = 6 and a 2 + b 2 = 50, find the value of ab.
Options: a. a. 7, b. b. 12, c. c. 15, d. d. 18, e. e. 20
Example Output: a

Example Input: Problem: the total age of a and b is 15 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. 66 years, e. 15 years
Example Output:
e