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: if a certain coin is flipped, the probability that the coin will land heads is 1 / 2. if the coin is flipped 6 times, what is the probability that it will land heads up on the first 4 flips but not on the last 2 flips ?
Options: a. 1 / 8, b. 1 / 16, c. 1 / 32, d. 1 / 64, e. 1 / 128
Example Output: d

Example Input: Problem: if jack walked 8 miles in 1 hour and 15 minutes, what was his rate of walking in miles per hour ?
Options: a. 4, b. 4.5, c. 6, d. 6.4, e. 15
Example Output: d

Example Input: Problem: sarah is driving to the airport. after driving at 15 miles per hour for one hour, she realizes that if she continues at that same average rate she will be an hour late for her flight. she then travels 60 miles per hour for the rest of the trip, and arrives 30 minutes before her flight departs. how many miles did she drive in all ?
Options: a. 45, b. 175, c. 210, d. 245, e. 280
Example Output:
a