Which of the Following Describes a Referrer Program

Which of the following describes a referrer program?

A) One site pays another site for the traffic it sends; traffic is directed in one direction.
B) A third party verifies referrals from one site to another site as traffic flows from one to the other.
C) One site pays several sites for traffic it sends; traffic is generated in multiple directions.
D) Referrals occur only after a company pays a commission for them; traffic is directed in one direction.