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: sakshi can do a piece of work in 5 days. tanya is 25 % more efficient than sakshi. the number of days taken by tanya to do the same piece of work :
Options: a. 4, b. 16, c. 18, d. 25, e. 10

Ex Output:
a


Ex Input:
Problem: dropbox charges $ 3.10 for first 1 / 5 of a month for 500 gb storage, plus $ 0.40 for each additional 1 / 5 of a month. what would dropbox charge for a data storage of 500 gb for 8 months ?
Options: a. $ 15.60, b. $ 16.00, c. $ 17.80, d. $ 18.70, e. $ 19.10

Ex Output:
d


Ex Input:
Problem: what is the greatest prime factor of 2 ^ 10 - 1 ?
Options: a. 17, b. 19, c. 23, d. 29, e. 31

Ex Output:

e