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: bill downloads the movierevenge of the avengersto his computer in 2.5 hours, using a download manager that downloads from 3 sources marked a, b and c. each source provides download at a constant rate, but the rates of different sources are not necessarily identical. if the movie was downloaded from sources a and c alone, it would take 4 hours to complete the download. the next day, source b is available, but the other sources are inactive. how long will it take to download the trailer of the movie, a file that is 10 times smaller, from source b alone ?
Options: a. 40 minutes, b. 15 minutes, c. 12 minutes, d. 10 minutes, e. 3 minutes
a