What is the purpose of Route Target Constraint?

A.
to avoid using route reflectors in MPLS VPN networks

B.
to avoid using multiple route distinguishers per VPN in MPLS VPN networks

C.
to be able to implement VPLS with BGP signaling

D.
to avoid sending unnecessary BGP VPNv4 or VPNv6 updates to the PE router

E.
to avoid BGP having to perform route refreshes