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.

[Q]: Problem: if a / b = 1 / 3, b / c = 2, c / d = 1 / 2, d / e = 3 and e / f = 1 / 10, then what is the value of abc / def ?
Options: a. 27 / 4, b. 27 / 8, c. 3 / 4, d. 3 / 20, e. 1 / 4
[A]: d


[Q]: Problem: find the value of 1 / ( 3 + 1 / ( 3 + 1 / ( 3 - 1 / 3 ) ) )
Options: a. 21 / 29, b. 27 / 89, c. 89 / 21, d. 29 / 21, e. none
[A]: b


[Q]: Problem: in what time will a train 100 meters long cross an electric pole, if its speed is 72 km / hr
Options: a. 5 seconds, b. 2.8 seconds, c. 3.5 seconds, d. 2.5 seconds, e. 2.6 seconds
[A]:
a