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.

Input: Consider Input: 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

Output: a

Input: Consider Input: Problem: a thief is spotted by a policeman from a distance of 150 meters. when the policeman starts the chase, the thief also starts running. if the speed of the thief be 8 km / hr and that of the policeman 10 km / hr, how far the thief will have run before he is overtaken ?
Options: a. 350 m, b. 200 m, c. 400 m, d. 600 m, e. none of them

Output: d

Input: Consider Input: Problem: at what rate percent per annum will the simple interest on a sum of money be 3 / 5 of the amount in 10 years ?
Options: a. 6 %, b. 7 %, c. 9 %, d. 3 %, e. 1 %
Output: a