what to the CE routers?

When implementing MPLS Layer 3 VPNs with customers running OSPF as the CE-PE routing
protocol, the service provider MPLS backbone looks like what to the CE routers?

A.
the backbone (Area 0)

B.
an external routing domain

C.
a superbackbone that is transparent to the CE OSPF routers

D.
a transit area (similar to a transit area for supporting virtual links)