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 jack walked 8 miles in 1 hour and 15 minutes, what was his rate of walking in miles per hour ?
Options: a. 4, b. 4.5, c. 6, d. 6.4, e. 15
[A]: d


[Q]: Problem: a bag contains 12 red marbles. if someone were to remove 2 marbles from the bag, one at a time, and replace the first marble after it was removed, the probability that neither marble would be red is 25 / 36. how many marbles are in the bag ?
Options: a. 24, b. 48, c. 60, d. 72, e. 84
[A]: b


[Q]: Problem: a train 250 m long is running with a speed of 68 kmph. in what time will it pass a man who is running at 8 kmph in the same direction in which the train is going ?
Options: a. 6 sec., b. 7 sec., c. 9 sec., d. 15 sec., e. none
[A]:
d