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 a mixture is 1 7 alcohol by volume and 2 7 water by volume, what is the ratio of the volume of alcohol to the volume of water in this mixture ?
Options: a. 4 / 2, b. 3 / 2, c. 1 / 2, d. 5 / 2, e. 7 / 2

Ex Output:
c

Ex Input:
Problem: what is the smallest integer that is multiple of 5, 711
Options: a. a. 70, b. b. 35, c. c. 385, d. d. 280, e. e. 140

Ex Output:
c

Ex Input:
Problem: 90 + 5 * 12 / ( 180 / 3 ) = ?
Options: a. 23, b. 78, c. 27, d. 91, e. 81

Ex Output:
d