You will be given a definition of a task first, then some input of the task.
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.

Problem: a boat can travel with a speed of 22 km / hr in still water. if the speed of the stream is 5 km / hr, find the time taken by the boat to go 108 km downstream
Options: a. 5 hours, b. 4 hours, c. 3 hours, d. 2 hours, e. none of these
Output:
b