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: if john makes a contribution to a charity fund at school, the average contribution size will increase by 50 % reaching $ 75 per person. if there were 1 other contributions made before john ' s, what is the size of his donation ?
Options: a. $ 100, b. $ 150, c. $ 200, d. $ 250, e. $ 450

Ex Output:
a

Ex Input:
Problem: by selling 20 pencils for a rupee a man loses 30 %. how many for a rupee should he sell in order to gain 30 % ?
Options: a. 8.56, b. 10.75, c. 14.5, d. 16.5, e. 11.4

Ex Output:
b

Ex Input:
Problem: a 300 m long train crosses a platform in 39 sec while it crosses a signal pole in 36 sec. what is the length of the platform ?
Options: a. 389 m, b. 350 m, c. 289 m, d. 299 m, e. 25 m

Ex Output:
e