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.

[EX Q]: Problem: excluding stoppages, the average speed of a bus is 60 km / hr and including stoppages, the average speed of the bus is 40 km / hr. for how many minutes does the bus stop per hour ?
Options: a. 18 min, b. 17 min, c. 19 min, d. 20 min, e. 21 min
[EX A]: d

[EX Q]: Problem: a 11 % stock yields 8 %. the market value of the stock is :
Options: a. rs. 72, b. rs. 137.50, c. rs. 112.50, d. rs. 116.50, e. none of these
[EX A]: b

[EX Q]: Problem: if john makes a contribution to a charity fund at school, the average contribution size will increase by 50 % reaching $ 75 per person. if there were 3 other contributions made before john ' s, what is the size of his donation ?
Options: a. $ 100, b. $ 150, c. $ 200, d. $ 250, e. $ 450
[EX A]:
b