Which of the following best illustrates the affiliate revenue model?

a$.$ eBay receives a small fee from a seller if a seller is successful in selling an

item.

b$.$ Epinions receives a fee after steering a customer to a participating Web

site where he or she makes a purchase.

c$.$ Flickr provides basic services for free, but charges a premium for

advanced services.

d$.$ Apple accepts micropayments for single music track downloads.