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.

Example Input: Problem: if 5 a = 3125, then the value of 5 ( a - 3 ) is :
Options: a. 25, b. 125, c. 625, d. 1625, e. none of these
Example Output: a

Example Input: Problem: a 10000 liter tank, half - full of water is being filled from a pipe with a flow rate of 1 kiloliter every 2 minutes. at the same time, the tank is losing water from two drains at a rate of 1 kiloliter every 4 minutes and every 6 minutes. how many minutes does it take to fill the tank completely ?
Options: a. 8, b. 12, c. 18, d. 24, e. 60
Example Output: e

Example Input: Problem: if a boat goes 7 km upstream in 42 minutes and the speed of the stream is 6 kmph, then the speed of the boat in still water is
Options: a. 12 kmph, b. 13 kmph, c. 14 kmph, d. 15 kmph, e. none of these
Example Output:
e