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: the average weight of a, b and c is 43 kg. if the average weight of a and b be 48 kg and that of b and c be 42 kg, then the weight of b is :
Options: a. 55 kg, b. 53 kg, c. 49 kg, d. 52 kg, e. 51 kg
Example Output: e

Example Input: Problem: if the population of a certain country increases at the rate of one person every 15 seconds, by how many persons does the population increase in 25 minutes ?
Options: a. 80, b. 100, c. 150, d. 240, e. 300
Example Output: b

Example Input: Problem: if a = 105 and a ^ 3 = 21 * 25 * 315 * b, what is the value of b ?
Options: a. 35, b. 7, c. 45, d. 49, e. 54
Example Output:
b