Detailed Instructions: 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.
Q: Problem: pipe a fills a tank of capacity 800 liters at the rate of 40 liters a minute. another pipe b fills the same tank at the rate of 30 liters a minute. a pipe at the bottom of the tank drains the tank at the rate of 20 liters a minute. if pipe a is kept open for a minute and then closed and pipe b is open for a minute and then closed and then pipe c is open for a minute and then closed and the cycle is repeated, when will the tank be full ?
Options: a. 48 minutes, b. 14 minutes, c. 39 minutes, d. 40 minutes 20 seconds, e. none of these
A:
a