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: a rope of which a calf is tied is increased from 12 m to 23 m, how much additional grassy ground shall it graze ?
Options: a. 2989, b. 2789, c. 1210, d. 1288, e. 1211

Ex Output:
c


Ex Input:
Problem: if a mixture is 1 ⁄ 7 alcohol by volume and 2 ⁄ 7 water by volume, what is the ratio of the volume of alcohol to the volume of water in this mixture ?
Options: a. 4 / 2, b. 3 / 2, c. 1 / 2, d. 5 / 2, e. 7 / 2

Ex Output:
c


Ex Input:
Problem: a train after traveling for 50 km meets with an accident and then proceeds at 3 / 4 of its former speed and arrives at its destination 35 minutes late. had the accident occurred 24 km farther, it would have reached the destination only 25 minutes late. what is the speed w of the train.
Options: a. a. 45, b. b. 33, c. c. 48, d. d. 55, e. e. 61

Ex Output:

c