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 greatest positive integer n such that 3 ^ n is a factor of 36 ^ 100 ?
Options: a. 100, b. 200, c. 300, d. 600, e. 900
Example Output: b

Example Input: Problem: two trains are moving in opposite directions at 60 km / hr and 90 km / hr. their lengths are 1.15 km and 0.6 km respectively. the time taken by the slower train to cross the faster train in seconds is ?
Options: a. 42, b. 9, c. 7, d. 67, e. 15
Example Output: a

Example Input: Problem: a train traveling at 72 kmph crosses a platform in 35 seconds and a man standing on the platform in 18 seconds. what is the length of the platform in meters ?
Options: a. 340 meters, b. 360 meters, c. 420 meters, d. 600 meters, e. can not be determined
Example Output:
a