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 person purchases 90 clocks and sells 40 clocks at a gain of 10 % and 50 clocks at a gain of 20 %. if he sold all of them at a uniform profit of 15 %, then he would have got rs. 40 less. the cost price of each clock is :
Options: a. 50, b. 90, c. 80, d. 28, e. 93

Ex Output:
c

Ex Input:
Problem: if ( 1 1.25 ) n = 4, then n =
Options: a. 400, b. 16, c. 4, d. 4, e. 400

Ex Output:
b

Ex Input:
Problem: a hall is 15 meters long and 10 meters wide. if the sum of the areas of the floor and the ceiling is equal to the sum of the areas of four walls, what is the volume of the hall ( in cubic meters ) ?
Options: a. 600, b. 700, c. 800, d. 900, e. 1000

Ex Output:
d