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: a fort had provision of food for 150 men for 45 days. after 10 days, 25 men left the fort. find out the number of days for which the remaining food will last.
Options: a. 44, b. 42, c. 40, d. 38, e. 36

Ex Output:
b


Ex Input:
Problem: the ratio of the volumes of two cubes is 1331: 2197. what is the ratio of their total surface areas ?
Options: a. 121: 169, b. 169: 122, c. 81: 124, d. 81: 126, e. 81: 129

Ex Output:
a


Ex Input:
Problem: a train covers a distance of 54 km in 15 min. if it takes 8 sec to pass a telegraph post, then the length of the train is ?
Options: a. 470, b. 480, c. 490, d. 500, e. 510

Ex Output:

b