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: the price of a book is increased from $ 300 to $ 390. what is the % of increase in its price ?
Options: a. 10 %, b. 20 %, c. 30 %, d. 35 %, e. 45 %
Example Output: c

Example Input: Problem: a hiker walked for 3 days. she walked 18 miles on the first day, walking 3 miles per hour. on the second day she walked for one less hour but she walked one mile per hour, faster than on the first day. on the third day 5 miles per hour for 6 hours. how many miles in total did she walk ?
Options: a. 68, b. 44, c. 58, d. 60, e. 62
Example Output: a

Example Input: Problem: the number of diagonals of a polygon of n sides is given by the formula v = n ( n - 3 ) / 2. if a polygon has twice as many diagonals as sides, how many sides does it have ?
Options: a. 3, b. 5, c. 6, d. 7, e. 8
Example Output:
b