What is the purpose of the Down bit in an OSPF LSA header?

What is the purpose of the Down bit in an OSPF LSA header?

A.
The Down bit is used to prevent redistribution loops between MP-BGP and OSPF.

B.
The Down bit is used to indicate the route that is being advertised has become unavailable.

C.
The Down bit is used to indicate an interface has been shut down or become unavailable.

D.
The Down bit is used to indicate the route that is being advertised is coming from an area that is lower in the hierarchical model.