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 walks at 10 kmph and 4 hours after his start, b cycles after him at 20 kmph. how far from the start does b catch up with a ?
Options: a. 100 km, b. 150 km, c. 80 km, d. 120 km, e. 200 km

Ex Output:
c


Ex Input:
Problem: the ratio of the volumes of two cubes is 1331: 2197. what is the ratio of their total surface areas ?
Options: a. 121: 169, b. 169: 122, c. 81: 124, d. 81: 126, e. 81: 129

Ex Output:
a


Ex Input:
Problem: the telephone company wants to add an area code composed of 2 letters to every phone number. in order to do so, the company chose a special sign language containing 524 different signs. if the company used 522 of the signs fully and two remained unused, how many additional area codes can be created if the company uses all 524 signs ?
Options: a. 46, b. 248, c. 2092, d. 15,128, e. 30,256

Ex Output:

c