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: a train 510 m long is running at a speed of 45 km / hr. in what time will it pass a bridge 140 m long ?
Options: a. 40, b. 52, c. 26, d. 21, e. 14
[EX A]: b

[EX Q]: Problem: if the average ( arithmetic mean ) of a and b is 45 and the average of b and c is 80, what is the value of c a ?
Options: a. 25, b. 60, c. 90, d. 140, e. it can not be determined from the information given.
[EX A]: b

[EX Q]: Problem: pipe a can fill a tank in 6 minutes and pipe b cam empty it in 24 minutes. if both the pipes are opened together after how many minutes should pipe b be closed, so that the tank is filled in 30 minutes ?
Options: a. 18, b. 27, c. 96, d. 27, e. 21
[EX A]:
c