Which of the following describes a referrer program?

A.
Referrals occur only after a company pays a commission for them; traffic is directed in one direction.

B.
One site pays another site for the traffic it sends; traffic is directed in one direction.

C.
A third party verifies referrals from one site to another site as traffic flows from one to the other.

D.
One site pays several sites for traffic it sends; traffic is generated in multiple directions.