when would it be required to leak routes into a Level 1 area?

When you design a network, when would it be required to leak routes into a Level 1 area?

A.
when a multicast RP is configured in the nonbackbone area

B.
when MPLS L3VPN PE devices are configured in the Level 1 areas

C.
when equal cost load balancing is required between the backbone and nonbackbone areas

D.
when unequal cost load balancing is required between the backbone and nonbackbone areas