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: if y exceeds x by 25 %, then x is less than y by ?
Options: a. 28 %, b. 21 %, c. 20 %, d. 30 %, e. 29 %
Example Output: c

Example Input: Problem: a bag contains 21 toys numbered 1 to 21. a toy is drawn and then another toy is drawn without replacement. find the probability that both toys will show even numbers.
Options: a. 5 / 21, b. 9 / 42, c. 6 / 22, d. 4 / 21, e. 3 / 21
Example Output: b

Example Input: Problem: 20 one identical machines, running continuously at the same constant rate, take 9 days to complete a shipment. how many additional machines, each running at the same constant rate, would be needed to reduce the time required to complete a shipment by two days ?
Options: a. 2, b. 3, c. 4, d. 6, e. 9
Example Output:
d