Instructions: 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.
Input: Problem: a runner runs the 40 miles from marathon to athens at a constant speed. halfway through the run she injures her foot, and continues to run at half her previous speed. if the second half takes her 8 hours longer than the first half, how many hours did it take the runner to run the second half ?
Options: a. 10, b. 12, c. 16, d. 20, e. 24
Output:
c