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: the size of a television screen is given as the length of the screen ' s diagonal. if the screens were flat, then the area of a square 19 - inch screen would be how many square inches greater than the area of a square 17 - inch screen ?
Options: a. 36, b. 4, c. 16, d. 38, e. 40
Example Output: a

Example Input: Problem: buses a and b start from a common bus stop x. bus a begins to travel in a straight line away from bus b at a constant rate of 20 miles per hour. one hour later, bus b begins to travel in a straight line in the exact opposite direction at a constant rate of 80 miles per hour. if both buses travel indefinitely, what is the positive difference, in minutes, between the amount of time it takes bus b to cover the exact distance that bus a has covered and the amount of time it takes bus b to cover twice the distance that bus a has covered ?
Options: a. 36, b. 72, c. 132, d. 144, e. 180
Example Output: c

Example Input: Problem: a man is 20 years older than his son. in two years, his age will be twice the age of his son. the present age of his son is
Options: a. 16, b. 14, c. 15, d. 18, e. 20
Example Output:
d