What is the role of OpenFlow in Software-Defined Networking (SDN)?

A.
It is the open standard that provides the application programming interfaces (APIs) that allow
SDN applications to interact with the SDN controller.

B.
It is one of the SDN applications that runs on the SDN Controller and allows you to view
existing “flows.”

C.
It is the open standard that establishes the abstraction layers in the SDN architecture.

D.
It is an open standard protocol that provides access to the provisioning mechanisms in network
devices.

Explanation:
http://h17007.www1.hp.com/docs/interopny/4AA4-3871ENW.pdf(page 7)