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: 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: ages of two persons differ by 20 years. if 6 year ago, the elder one be 3 times as old the younger one, find their present age
Options: a. 12,28, b. 14,30, c. 16,36, d. 18,34, e. 19,34

Ex Output:
c

Ex Input:
Problem: the ratio of laxmi ' s age to the age of her mother is 3: 11. the difference of their ages is 24 years. the ratio of their ages after 3 years will be ?
Options: a. 1: 9, b. 1: 3, c. 1: 2, d. 1: 1, e. 3: 3

Ex Output:
b