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 train travels at the rate of 10 miles / hr for the first hour of a trip, at 20 miles / hr for the second hour, at 30 miles / hr for the third hour and so on. how many hours will it take the train to complete a 360 - mile journey ? assume that the train makes no intermediate stops.
Options: a. 8, b. 8.5, c. 9, d. 9.5, e. 10
Output:
a