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: when positive integer n is divided by 3, the remainder is 1. when n is divided by 7, the remainder is 5. what is the smallest positive integer k such that k + n is a multiple of 21 ?
Options: a. 2, b. 4, c. 6, d. 8, e. 10

Ex Output:
a


Ex Input:
Problem: a train running at the speed of 20 km / hr crosses a post in 10 seconds. what is the length of the train ?
Options: a. 150, b. 250, c. 415, d. 200, e. 312

Ex Output:
d


Ex Input:
Problem: a train covers a distance of 100 km in 1 hour. if its speed is decreased by 12 km / hr, the time taken by the car to cover the same distance will be ?
Options: a. 1 hr, b. 1 hr 8 min, c. 50 min, d. 1 hr 30 min, e. 1 hr 45 min

Ex Output:

b