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: walking at 4 / 5 of her normal speed, a worker is 10 minutes later than usual in reaching her office. the usual time ( in minutes ) taken by her to cover the distance between her home and her office is
Options: a. 30, b. 35, c. 40, d. 45, e. 50

Ex Output:
c


Ex Input:
Problem: when positive integer n is divided by 7, the remainder is 1. when n is divided by 11, the remainder is 5. what is the smallest positive integer k such that k + n is a multiple of 77 ?
Options: a. 2, b. 4, c. 6, d. 8, e. 10

Ex Output:
c


Ex Input:
Problem: two trains, one from howrah to patna and the other from patna to howrah, start simultaneously. after they meet, the trains reach their destinations after 25 hours and 81 hours respectively. the ratio of their speeds is :
Options: a. 2: 3, b. 4: 3, c. 9: 5, d. 9: 16, e. none of these

Ex Output:

c