What is used to provide virtual intra-area OSPF connectivity across MPLS VPN backbone so the customer’s traffic will prefer the MPLS VPN backbone over a backdoor OSPF link between the customer’s sites?

A.
OSPF virtual-link

B.
OSPF sham-link

C.
OSPF nssa-link

D.
OSPF stub-link

E.
OSPF demand-circuit

F.
OSPF LSA1-link