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: the average age of 17 persons in a office is 15 years. out of these, the average age of 5 of them is 14 years and that of the other 9 persons is 16 years. the age of the 15 th person is ?
Options: a. 39, b. 41, c. 45, d. 42, e. 50

Ex Output:
b

Ex Input:
Problem: a train 510 m long is running at a speed of 45 km / hr. in what time will it pass a bridge 140 m long ?
Options: a. 40, b. 52, c. 26, d. 21, e. 14

Ex Output:
b

Ex Input:
Problem: a 600 meter long train crosses a signal post in 40 seconds. how long will it take to cross a 7.2 kilometer long bridge, at the same speed ?
Options: a. 4 min, b. 2 min, c. 8 min, d. 9 min, e. 3 min

Ex Output:
c