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 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
[EX A]: a

[EX Q]: Problem: the sum of 4 consecutive even numbers is 140. what would be the smallest number ?
Options: a. 32, b. 88, c. 70, d. 123, e. 12
[EX A]: a

[EX Q]: Problem: if 4 people are selected from a group of 8 married couples, what is the probability that none of them would be married to each other ?
Options: a. 1 / 33, b. 2 / 33, c. 8 / 13, d. 16 / 33, e. 11 / 12
[EX A]:
c