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.

Ex Input:
Problem: 1391 x 1391 = ?
Options: a. a. 1951609, b. b. 1951601, c. c. 1951602, d. d. 1951603, e. e. 1934881

Ex Output:
e

Ex 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

Ex Output:
a

Ex Input:
Problem: how many different ways can 2 students be seated in a row of 5 desks, so that there is always at least one empty desk between the students ?
Options: a. 2, b. 3, c. 4, d. 6, e. 12

Ex Output:
e