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 finish a work in 60 days, b in 9 days and c in 2 days, b and c start the work but are forced to leave after 3 days. the remaining work was done by a in ?
Options: a. 10 days, b. 25 days, c. 6 days, d. 7 days, e. 8 days
Example Output: b

Example Input: Problem: by selling 20 pencils for a rupee a man loses 30 %. how many for a rupee should he sell in order to gain 30 % ?
Options: a. 8.56, b. 10.75, c. 14.5, d. 16.5, e. 11.4
Example Output: b

Example Input: Problem: the number of boxes in a warehouse can be divided evenly into 7 equal shipments by boat or 28 equal shipments by truck. what is the smallest number of boxes that could be in the warehouse ?
Options: a. 28, b. 33, c. 54, d. 81, e. 162
Example Output:
a