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: a mixture of 70 litres of milk and water contains 10 % water. how many litres of water should be added to the mixture so that the mixture contains 12 1 / 2 % water ?
Options: a. 2, b. 8, c. 6, d. 3, e. 4

Ex Output:
a


Ex Input:
Problem: find the constant k so that: - x 2 - ( k + 10 ) x - 8 = - ( x - 2 ) ( x - 4 )
Options: a. 11, b. 12, c. 16, d. 14, e. 15

Ex Output:

c