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 purchase price of an article is $ 48. in order to include 5 % of cost for overhead and to provide $ 12 of net profit, the markup should be
Options: a. 15 %, b. 25 %, c. 30 %, d. 40 %, e. 45 %
Example Output: c

Example Input: Problem: a sum of money at simple interest amounts to rs. 720 after 2 years and to rs. 1020 after a further period of 5 years. the sum is :
Options: a. 400, b. 500, c. 600, d. 700, e. 800
Example Output: c

Example Input: Problem: two trains of length 100 m and 200 m are 100 m apart. they start moving towards each other on parallel tracks, at speeds 36 kmph and 72 kmph. in how much time will the trains cross each other ?
Options: a. 40 / 6, b. 40 / 3, c. 40 / 9, d. 40 / 2, e. 40 / 1
Example Output:
b