Which two OSPF network scenarios require OSPF virtual link configuration? (Choose two.)

A.
to connect an OSPF non-backbone area to area 0 through another non-backbone area

B.
to connect an NSSA area to an external routing domain

C.
to connect two parts of a partitioned backbone area through a non-backbone area

D.
to enable route leaking from Level 2 into Level 1

E.
to enable route leaking from Level 1 into Level 2

F.
to enable OSPF traffic engineering