What is the best supportive evidence to use in an argument aiming to prove that declining CD sales are the result of illegal music downloading?
A. an interview with a person who works as a cashier at a popular music store
B. an interview with a person who prefers to buy and download music on the Internet
C. data that compares the number of CDs sold to the number of songs downloaded illegally
D. data that tells the reader the number of CDs sold in each of the past ten years