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.

Input: Consider Input: Problem: a cyclist bikes x distance at 8 miles per hour and returns over the same path at 10 miles per hour. what is the cyclist ' s average rate for the round trip in miles per hour ?
Options: a. 8.1, b. 8.9, c. 8.6, d. 8.3, e. 9.0

Output: b

Input: Consider Input: Problem: a mixture of 70 litres of milk and water contains 10 % water. how many litres of water should be added to the mixture so that the mixture contains 12 1 / 2 % water ?
Options: a. 2, b. 8, c. 6, d. 3, e. 4

Output: a

Input: Consider Input: 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
Output: a