$13.$ Which of the following best illustrates the transaction fee 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