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: two taps a and b can fill a cistern in 12 minutes and 18 minutes respectively. they are turned on at the same time. if the tap a is turned off after 4 minutes, how long will tap b take to fill the rest of the cistern ?
Options: a. 8 min., b. 9 min., c. 10 min., d. 7 min., e. none of these
[A]: a

[Q]: Problem: trapezoid jklm in the x - y plane has coordinates j = ( 3, 4 ), k = ( 3, 1 ), l = ( 5, 7 ), and m = ( 5, 4 ). what is its perimeter ?
Options: a. 34, b. 36, c. 38, d. 40, e. ( f ) 42
[A]: a

[Q]: Problem: a train 124 m long running at 68 kmph crosses a platform in 19 sec. what is the length of the platform ?
Options: a. 338, b. 234, c. 240, d. 881, e. 271
[A]:
b