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 l. c. m of two numbers is 495 and their h. c. f is 5. if the sum of the numbers is 100, then their difference is
Options: a. 19, b. 10, c. 28, d. 27, e. 29
Example Output: b

Example Input: Problem: in a group of 100 people, 90 have an age of more 30 years, and the others have an age of less than 20 years. if a person is selected at random from this group, what is the probability the person ' s age is less than 20 ?
Options: a. 0.1, b. 0.55, c. 0.65, d. 0.75, e. 0.85
Example Output: a

Example Input: Problem: zinc and copper are melted together in the ratio 9: 11. what is the weight of melted mixture, if 26.1 kg of zinc has been consumed in it ?
Options: a. 58 kg, b. 60 kg, c. 64 kg, d. 70 kg, e. none
Example Output:
a