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: what is the sum of natural numbers between 30 and 100
Options: a. 4615, b. 4860, c. 5000, d. 5500, e. 5550
Example Output: a

Example Input: Problem: what is the greatest positive integer n such that 3 ^ n is a factor of 36 ^ 100 ?
Options: a. 100, b. 200, c. 300, d. 600, e. 900
Example Output: b

Example Input: Problem: two numbers are in the ratio of 7: 6. if 18 be subtracted from each, they are in the ratio of 6: 5. find the numbers ?
Options: a. 5,9, b. 7,9, c. 4,7, d. 7,6, e. 6,9
Example Output:
d