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: buses a and b start from a common bus stop x. bus a begins to travel in a straight line away from bus b at a constant rate of 20 miles per hour. one hour later, bus b begins to travel in a straight line in the exact opposite direction at a constant rate of 80 miles per hour. if both buses travel indefinitely, what is the positive difference, in minutes, between the amount of time it takes bus b to cover the exact distance that bus a has covered and the amount of time it takes bus b to cover twice the distance that bus a has covered ?
Options: a. 36, b. 72, c. 132, d. 144, e. 180
[A]: c


[Q]: Problem: walking at 4 / 5 of her normal speed, a worker is 10 minutes later than usual in reaching her office. the usual time ( in minutes ) taken by her to cover the distance between her home and her office is
Options: a. 30, b. 35, c. 40, d. 45, e. 50
[A]: c


[Q]: Problem: in a house a hall is 20 m long, 15 m wide and 5 m high. its interior has to be covered with mat. what will be the total expenditure if it costs rs. 10 per square m ?
Options: a. s. 9500, b. s. 50000, c. s. 57000, d. s. 60000, e. s. 62000
[A]:
a