Which four statements about the Carrier Supporting Carrier （CSC） feature are true？ （）

A.MPLS is required between the PE and CE routers that connect the backbone carrier to the customer carrier.
B.The backbone carrier must enable the PE router to check that the packets it receives from the CErouter contain only the labels that the PE router advertised to the CE router.
C.Every packet that crosses the backbone carrier must be encapsulated so that the packet includes MPLS labels
D.AII PE routers that link the backbone carrier to the customer carrier must run Route Target Rewrite.
E.Arouting protocol is required between the PE and CE routers that connect the backbone carrierto thecustomer carrier. The routing protocol enables the customer carrier to exchange IGP routing information with the backbone carrier.