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: the average of first five multiples of 7 is :
Options: a. 9, b. 16, c. 15, d. 8, e. 21
[A]: e

[Q]: Problem: if the area of a circle decreases by 66 %, then the radius of a circle decreases by
Options: a. 20 %, b. 18 %, c. 36 %, d. 64 %, e. none of these
[A]: e

[Q]: Problem: find the principle on a certain sum of money at 5 % per annum for 3 1 / 5 years if the amount being rs. 1160 ?
Options: a. rs. 1000, b. rs. 1100, c. rs. 1010, d. rs. 10000, e. none of these
[A]:
a