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 man sitting in a train which is traveling at 36 kmph observes that a goods train, traveling in opposite direction, takes 10 seconds to pass him. if the goods train is 240 m long, find its speed
Options: a. 50 kmph, b. 56 kmph, c. 58 kmph, d. 62 kmph, e. 72 kmph
Example Output: a

Example Input: Problem: a cement mixture is composed of 3 elements. by weight, 1 / 2 of the mixture is sand, 1 / 5 of the mixture is water, and the remaining 15 pounds of the mixture is gravel. what is the weight of the entire mixture in pounds ?
Options: a. 30, b. 40, c. 50, d. 60, e. 80
Example Output: c

Example Input: Problem: from the sale of sleeping bags, a retailer made a gross profit of 16 % of the wholesale cost. if each sleeping bag was sold for $ 28, what was the wholesale cost per bag ?
Options: a. 3.0, b. 3.36, c. 24.13, d. 25.0, e. 31.36
Example Output:
c