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.

Example Input: Problem: a man ' s speed with the current is 18 km / hr and the speed of the current is 2.5 km / hr. the man ' s speed against the current is ?
Options: a. 10, b. 18, c. 50, d. 30, e. 40
Example Output: b

Example Input: 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
Example Output: e

Example Input: Problem: the ratio, by volume, of soap to alcohol to water in a certain solution is 20: 50: 80. the solution will be altered so that the ratio of soap to alcohol is doubled while the ratio of soap to water is halved. if the altered solution will contain 100 cubic centimeters of alcohol, how many cubic centimeters of water will it contain ?
Options: a. 520, b. 640, c. 700, d. 750, e. 800
Example Output:
b