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 cyclist bikes x distance at 8 miles per hour and returns over the same path at 10 miles per hour. what is the cyclist ' s average rate for the round trip in miles per hour ?
Options: a. 8.1, b. 8.9, c. 8.6, d. 8.3, e. 9.0
Example Output: b

Example Input: Problem: a 24 ' ' x 24 ' ' square metal plate needs to be fixed by a carpenter on to a wooden board. the carpenter uses nails all along the edges of the square such that there are 25 nails on each side of the square. each nail is at the same distance from the neighboring nails. how many nails does the carpenter use ?
Options: ['a. 94', 'b. 95', 'c. 96', 'd. 97', 'e. 98']
Example Output: c

Example Input: Problem: a car salesman earns a base salary of $ 1000 per month plus a commission of $ 200 for each car he sells. if the car salesman earned $ 2000 in march, how many cars does he need to sell in april in order to double his march earnings ?
Options: a. 14, b. 15, c. 16, d. 17, e. 18
Example Output:
b