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 goods train runs at the speed of 72 kmph and crosses a 250 m long platform in 24 seconds. what is the length of the goods train ?
Options: a. 230 m, b. 240 m, c. 260 m, d. 270 m, e. none of these
[EX A]: a

[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: how many seconds will a 300 m long train take to cross a man walking with a speed of 3 km / hr in the direction of the moving train if the speed of the train is 63 km / hr ?
Options: a. 12 sec, b. 30 sec, c. 86 sec, d. 15 sec, e. 18 sec
[EX A]:
e