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 age of a family of 5 members is 20 years. if the age of the youngest member be 10 years then what was the average age of the family at the time of the birth of the youngest member ?
Options: a. 13.5, b. 14, c. 15, d. 12.5, e. none
Example Output: d

Example Input: Problem: a shopkeeper sells 500 metres of cloth for rs. 18000 at a loss of rs. 5 per metre. find his cost price for one metre of cloth ?
Options: a. 12, b. 27, c. 29, d. 50, e. 41
Example Output: e

Example Input: Problem: water consists of hydrogen and oxygen, and the approximate ratio, by mass, of hydrogen to oxygen is 2: 16. approximately how many grams of hydrogen are there in 117 grams of water ?
Options: a. 7, b. 9, c. 11, d. 13, e. 15
Example Output:
d