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.

Problem: a person buys an article at $ 600. at what price should he sell the article so as to make a profit of 8 % ?
Options: a. 670, b. 689, c. 648, d. 740, e. 889
c

Problem: a certain protective additive increases from 50 days to 60 days the time between required maintenance checks on an industrial vehicle. by what percent is the time between maintenance checks increased by using the additive ?
Options: a. 25 %, b. 33 1 / 3 %, c. 50 %, d. 66 2 / 3 %, e. 20 %
e

Problem: how many times digit 5 is used while writing numbers from 1 to 500 ?
Options: a. 103, b. 98, c. 94, d. 105, e. 100
e