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 Q]: Problem: mr. loyd wants to fence his square shaped land of 120 sqft each side. if a pole is laid every 12 ft how many poles do he need ?
Options: a. 20, b. 30, c. 40, d. 50, e. 60
[EX A]: c

[EX Q]: Problem: when positive integer n is divided by 3, the remainder is 1. when n is divided by 7, the remainder is 5. what is the smallest positive integer k such that k + n is a multiple of 21 ?
Options: a. 2, b. 4, c. 6, d. 8, e. 10
[EX A]: a

[EX Q]: Problem: the average age of an adult class is 40 years. 12 new students with an avg age of 32 years join the class. therefore decreasing theaverage by 4 year. find what was theoriginal strength of class ?
Options: a. 11, b. 12, c. 17, d. 19, e. 21
[EX A]:
b