A Tier 1 service provider is implementing a backbone network to support connectivity for multiple Tier 2 service provide

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A Tier 1 service provider is implementing a backbone network to support connectivity for multiple Tier 2 service providers, by using a Carrier Supporting Carrier design. Each Tier 2 service provider requires the ability to implement MPLS through the Tier 1 backbone. What occurs with respect to traffic for the Tier 2 providers?

A. The Tier 2 provider uses MPLS over GRE tunnels for each customer VRF to create an LSP through the Tier 1 backbone.
B. Connectivity between Tier 2 providers is established via a full mesh of MPLS L2VPN circuits.
C. Tier 2 traffic across the backbone network has an additional label pushed onto the existing MPLS label stack.
D. The Tier 1 provider implements each Tier 2 provider into the backbone via BGP into the default global routing table.