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 trains travel in opposite directions at 36 kmph and 45 kmph and a man sitting in slower train passes the faster train in 8 seconds. the length of the faster train is
Options: a. 80 m, b. 100 m, c. 120 m, d. 180 m, e. 160 m
[EX A]: b

[EX Q]: Problem: a man swims downstream 36 km and upstream 18 km taking 3 hours each time, what is the speed of the man in still water ?
Options: a. 7, b. 8, c. 9, d. 2, e. 4
[EX A]: c

[EX Q]: Problem: a certain fraction is equivalent to 2 / 5. if the numerator of the fraction is increased by 4 and the denominator is doubled, the new fraction is equivalent to 1 / 3. what is the difference of the numerator and denominator of the original fraction ?
Options: a. 49, b. 9, c. 28, d. 26, e. 21
[EX A]:
b