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.

Ex Input:
Problem: if a certain coin is flipped, the probability that the coin will land heads is 1 / 2. if the coin is flipped 6 times, what is the probability that it will land heads up on the first 4 flips but not on the last 2 flips ?
Options: a. 1 / 8, b. 1 / 16, c. 1 / 32, d. 1 / 64, e. 1 / 128

Ex Output:
d


Ex Input:
Problem: if a sum of money doubles itself in 20 years at simple interest, the ratepercent per annum is
Options: a. 12, b. 12.5, c. 13, d. 5, e. 14

Ex Output:
d


Ex Input:
Problem: the difference of two numbers is 2 and the difference of their squares is 12. the sum of the number is
Options: a. 6, b. 8, c. 10, d. 22, e. 24

Ex Output:

a