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: the speed of the boat in still water in 12 kmph. it can travel downstream through 54 kms in 3 hrs. in what time would it cover the same distance upstream ?
Options: a. 8 hours, b. 6 hours, c. 4 hours, d. 5 hours, e. 9 hours
e

Problem: 50 litres of diesel is required to travel 600 km using a 800 cc engine. if the volume of diesel required to cover a distance varies directly as the capacity of the engine, then how many litres of diesel is required to travel 800 km using 1200 cc engine ?
Options: a. 80 litres, b. 90 litres, c. 100 litres, d. 170 litres, e. none of these
c