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: a walks at 10 kmph and 4 hours after his start, b cycles after him at 20 kmph. how far from the start does b catch up with a ?
Options: a. 100 km, b. 150 km, c. 80 km, d. 120 km, e. 200 km
Example Output: c

Example Input: Problem: a student travels from his house to school at 12 km / hr and reaches school 2 hours late. the next day he travels 20 km / hr and reaches school 1 hour early. what is the distance between his house and the school ?
Options: a. 90, b. 100, c. 110, d. 120, e. 130
Example Output: a

Example Input: Problem: the two lines y = x and x = - 4 intersect on the coordinate plane. if z represents the area of the figure formed by the intersecting lines and the x - axis, what is the side length e of a cube whose surface area is equal to 6 z ?
Options: a. e = 16, b. e = 8 √ 2, c. e = 8, d. e = 2 √ 2, e. ( √ 2 ) / 3
Example Output:
d