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: two trains of equal length are running on parallel lines in the same directions at 46 km / hr. and 36 km / hr. the faster trains pass the slower train in 72 seconds. the length of each train is :
Options: a. 82 m, b. 50 m, c. 72 m, d. 100 m, e. none of these
Example Output: d

Example Input: Problem: in a group of cows and chickens, the number of legs was 12 more than twice the number of heads. the number of cows was :
Options: a. 6, b. 7, c. 10, d. 12, e. 14
Example Output: a

Example Input: Problem: if the arithmetic mean of p and q is 10 and the arithmetic mean of q and r is 22, what is the value of r - p ?
Options: a. 20, b. 10, c. 30, d. 24, e. 5
Example Output:
d