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: what is the units digit of ( 3 ^ 13 ) ( 4 ^ 13 ) ?
Options: a. 2, b. 4, c. 6, d. 7, e. 8
Example Output: a

Example Input: Problem: the present population of a town is 2000. population increase rate is 20 % p. a. find the population of town after 1 years ?
Options: a. 1000, b. 2400, c. 1500, d. 1600, e. 1250
Example Output: b

Example Input: Problem: the average of the marks of 25 students in a class is 70. if the marks of each student are doubled, find the new average ?
Options: a. 140, b. 100, c. 85, d. 160, e. 190
Example Output:
a