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 the stoppages, the speed of a bus is 48 km / hr and including the stoppages the speed of the bus is 12 km / hr. for how many minutes does the bus stop per hour ?
Options: a. 45 min, b. 10 min, c. 12 min, d. 20 min, e. 18 min

Ex Output:
a


Ex Input:
Problem: if a person walks at 25 km / hr instead of 10 km / hr, he would have walked 20 km more. the actual distance traveled by him is :
Options: a. 50 km, b. 56 km, c. 13.3 km, d. 70 km, e. 80 km

Ex Output:
c


Ex Input:
Problem: the average mark of the students of a class in a particular exam is 80. if 5 students whose average mark in that exam is 60 are excluded, the average mark of the remaining will be 90. find the number of students who wrote the exam.
Options: a. 22, b. 27, c. 15, d. 99, e. 21

Ex Output:

c