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: two trains of equal are running on parallel lines in the same direction at 45 km / hr and 36 km / hr. the faster train passes the slower train in 36 sec. the length of each train is ?
Options: a. 45, b. 88, c. 66, d. 55, e. 22

Ex Output:
a

Ex Input:
Problem: a man traveled from the village to the post - office at the rate of 25 kmph and walked back at the rate of 4 kmph. if the whole journey took 5 hours 48 minutes, find the distance of the post - office from the village ?
Options: a. 70 m, b. 80 km, c. 20 km, d. 10 km, e. 73 km

Ex Output:
c

Ex Input:
Problem: a fair 2 sided coin is flipped 4 times. what is the probability that tails will be the result at least twice, but not more than 4 times ?
Options: a. 1 / 8, b. 2 / 3, c. 5 / 8, d. 4 / 7, e. 3 / 7

Ex Output:
c