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: in what time will a railway train 50 m long moving at the rate of 36 kmph pass a telegraph post on its way ?
Options: a. 5 sec, b. 4 sec, c. 3 sec, d. 6 sec, e. 1 sec
Example Output: a

Example Input: Problem: a train which has 430 m long, is running 45 kmph. in what time will it cross a person moving at 9 kmph in same direction ?
Options: a. 56 sec, b. 43 sec, c. 36 sec, d. 29 sec., e. 19 sec.
Example Output: b

Example Input: Problem: a man & his wife appear in an interview for 2 vacancies in the same post. the probability of husband ' s selection is 1 / 7 & the probability of wife ' s selection is 1 / 5. what is the probability that only 1 of them is selected ?
Options: a. 1 / 3, b. 2 / 3, c. 2 / 7, d. 3 / 11, e. 4 / 5
Example Output:
c