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: two pipes a and b can fill a tank in 36 hours and 46 hours respectively. if both the pipes are opened simultaneously, how much time will be taken to fill the tank ?
Options: a. 20.195 hrs, b. 21.195 hrs, c. 22.195 hrs, d. 23.195 hrs, e. 20.495 hrs
[EX A]: a

[EX Q]: Problem: if the population of a certain country increases at the rate of one person every 15 seconds, by how many persons does the population increase in 25 minutes ?
Options: a. 80, b. 100, c. 150, d. 240, e. 300
[EX A]: b

[EX Q]: Problem: a train 360 m long is running at a speed of 45 km / hr. in what time will be pass a bridge 140 m long ?
Options: a. 40 sec, b. 42 sec, c. 45 sec, d. 48 sec, e. none
[EX A]:
a