Definition: 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 rower can row 7 km / h in still water. when the river is running at 2 km / h, it takes the rower 1 hour to row to big rock and back. how many kilometers is it to big rock ?
Options: a. 2.92, b. 3.04, c. 3.12, d. 3.21, e. 3.35
Output:
d