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: a can do a job in 15 days and b in 20 days. if they work on it together for 4 days, then the fraction of the work that is left is ?
Options: a. 2 / 15, b. 8 / 15, c. 3 / 11, d. 1 / 12, e. 6 / 13
Example Output: b

Example Input: Problem: if each participant of a chess tournament plays exactly one game with each of the remaining participants, then 276 games will be played during the tournament. find the number of participants.
Options: a. 20, b. 22, c. 24, d. 26, e. 28
Example Output: c

Example Input: Problem: 8 men, working 8 hours a day can complete a work in 18 days. how many hours a day must 12 men work to complete the same work in 12 days ?
Options: a. 6, b. 7, c. 8, d. 9, e. 10
Example Output:
c