Given the task definition and input, reply with output. 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 pump will fill a tank in 3 hours. because of a leak in the tank, it took 3 hours 30 min to fill the tank. in what time the leak can drain out all the water of the tank and will make tank empty ?
Options: a. 20 hrs, b. 21 hrs, c. 22 hrs, d. 23 hrs, e. 24 hrs
b