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: what is the median of a set of consecutive integers if the sum of nth number from the beginning and nth number from the end is 110 ?
Options: a. 10, b. 25, c. 55, d. 75, e. 100

Ex Output:
c


Ex Input:
Problem: an optometrist charges $ 150 per pair for soft contact lenses and $ 85 per pair for hard contact lenses. last week she sold 5 more pairs of soft lenses than hard lenses. if her total sales for pairs of contact lenses last week were $ 1,220, what was the total number of pairs of contact lenses that she sold ?
Options: a. 9, b. 13, c. 15, d. 17, e. 19

Ex Output:
a


Ex Input:
Problem: if rs. 10 be allowed as true discount on a bill of rs. 110 due at the end of a certain time, then the discount allowed on the same sum due at the end of double the time is :
Options: a. rs. 20, b. rs. 21.81, c. rs. 22, d. rs. 18.33, e. none

Ex Output:

d