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: what is the total number of integers between 200 and 500 that are divisible by 25 ?
Options: a. 20, b. 25, c. 14, d. 15, e. 18
Example Output: d

Example Input: Problem: what is the probability of getting a queen or spade from a set of cards ?
Options: a. 3 / 13, b. 4 / 13, c. 5 / 13, d. 6 / 13, e. 7 / 13
Example Output: b

Example Input: Problem: two trains of equal are running on parallel lines in the same direction at 18 km / hr and 36 km / hr. the faster train passes the slower train in 36 sec. the length of each train is ?
Options: a. 25, b. 26, c. 27, d. 28, e. 21
Example Output:
a