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: a worker is paid a regular rate of rs. 30 for completing a survey. the worker completes 100 surveys per week. for any survey involving the use of her cellphone, she is paid at a rate of that is 20 % higher than her regular rate. if she completed 50 surveys involving the use of her cellphone, how much did she get that week ?
Options: a. 2200, b. 1100, c. 2500, d. 2800, e. 3300

Ex Output:
e

Ex 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

Ex Output:
e