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.

[Q]: Problem: what is the units digit of ( 3 ^ 13 ) ( 4 ^ 13 ) ?
Options: a. 2, b. 4, c. 6, d. 7, e. 8
[A]: a

[Q]: Problem: the population of a town increases 25 % and 40 % respectively in two consecutive years. after the growth the present population of the town is 1750. then what is the population of the town 2 years ago ?
Options: a. a. 750, b. b. 800, c. c. 850, d. d. 900, e. e. 950
[A]: b

[Q]: Problem: of the 200 stamps in a collection, 80 are foreign and 50 are more than 10 years old. if 20 stamps are both foreign and more than 10 years old, how many stamps are neither foreign nor more than 10 years old ?
Options: a. 60, b. 90, c. 100, d. 130, e. 150
[A]:
b