Help..

A technology company has hired an advertising agency to determine the popularity of the company’s new MP3 player. Which method should the agency use to collect information on the item’s popularity?
A. Pay 25 people to be part of a focus group to discuss the features of the product. B. Ask all of the sales representatives of the company.
C. Choose a simple random sample from people who have bought the product. D. Count the number of sales of the product in stores that sell the product.