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: one side of a rectangular field is 14 m and one of its diagonal is 17 m. find the area of the field.
Options: a. 134.4, b. 120, c. 150, d. 180, e. none
Example Output: a

Example Input: Problem: a van takes 5 hours to cover a distance of 495 km. what speed in kph should the van maintain to cover the same distance in 3 / 2 of the previous time ?
Options: a. 60, b. 63, c. 66, d. 69, e. 72
Example Output: c

Example Input: Problem: sand is poured into a box so that the box is being filled at the rate of 4 cubic feet per hour. if the empty rectangular box is 7 feet long, 6 feet wide, and 2 feet deep, approximately how many hours does it take to fill the box ?
Options: ['a. 20', 'b. 21', 'c. 22', 'd. 23', 'e. 24']
Example Output:
b