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: excluding stoppages, the average speed of a bus is 60 km / hr and including stoppages, the average speed of the bus is 40 km / hr. for how many minutes does the bus stop per hour ?
Options: a. 18 min, b. 17 min, c. 19 min, d. 20 min, e. 21 min

Ex Output:
d


Ex Input:
Problem: if 5 a = 3125, then the value of 5 ( a - 3 ) is :
Options: a. 25, b. 125, c. 625, d. 1625, e. none of these

Ex Output:
a


Ex Input:
Problem: if a ^ 2 + b ^ 2 = 144 and ab ≠ 0, then the greatest possible value for b is between
Options: a. 16 and 13, b. 12 and 3, c. 11 and 4, d. 10 and 5, e. 9 and 6

Ex Output:

b