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.

Problem: if 7 a = 8 b and ab ≠ 0, what is the ratio of a / 8 to b / 7 ?
Options: a. 16 / 14, b. 8 / 7, c. 1, d. 7 / 8, e. 64 / 49
c

Problem: an engine moves at the speed of 30 kmph without any coaches attached to it. speed of the train reduces at the rate that varies directly as the square root of the number of coaches attached. when 9 coaches are attached speed decreases to 18 kmph. what will be the speed of train when 16 coaches are attached.
Options: a. 14, b. 16, c. 18, d. 20, e. 22
a

Problem: how many prime numbers are between 29 / 3 and 87 / 6 ?
Options: a. 1, b. 2, c. 3, d. 4, e. 5

b