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 man can row 24 kmph in still water. it takes him thrice as long to row up as to row down the river. find the rate of the stream ?
Options: a. 6, b. 12, c. 18, d. 15, e. 20
Output:
b