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: what is the greatest positive integer n such that 3 ^ n is a factor of 36 ^ 100 ?
Options: a. 100, b. 200, c. 300, d. 600, e. 900
[EX A]: b

[EX Q]: Problem: a certain sum is invested at simple interest at 18 % p. a. for two years instead of investing at 12 % p. a. for the same time period. therefore the interest received is more by rs. 300. find the sum ?
Options: a. 7000, b. 2500, c. 2778, d. 2800, e. 2791
[EX A]: b

[EX Q]: Problem: visitors to show were charged rs. 15 each on the first day. rs. 7.50 on the second day, rs. 2.50 on the third day and total attendance on the 3 days were in ratio 2: 5: 13 respectively. the average charge per person for the whole show is ?
Options: a. 1, b. 4, c. 5, d. 6, e. 8
[EX A]:
c