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 movie buff buys movies on dvd and on blu - ray in a ratio of 13: 4. if she returns 2 blu - ray movies, that ratio would change to 7: 2. if she buys movies on no other medium, what was the original number of movies purchased ?
Options: a. 22, b. 28, c. 77, d. 99, e. 119

Ex Output:
e


Ex Input:
Problem: the average of first 10 natural numbers is ?
Options: a. 5.6, b. 5.5, c. 5.1, d. 5.8, e. 5.2

Ex Output:
b


Ex Input:
Problem: if a 1 mm thick paper is folded so that the area is halved at every fold, then what would be the thickness of the pile after 50 folds ?
Options: ['a. 100 km', 'b. 1000 km', 'c. 1 million km', 'd. 1 billion km', 'e. none']

Ex Output:

d