How does an MPLS Layer 3 VPN function?

How does an MPLS Layer 3 VPN function?
A. set of sites use multiprotocol BGP at the customer site for aggregation
B. multiple customer sites interconnect through service provider network to create secure tunnels between customer edge devices
C. set of sites interconnect privately over the Internet for security
D. multiple customer sites interconnect through a service provider network using customer edge to provider edge connectivity