Which two statements describe the scenario when a router will send out a Type 4 link-state update packet to its neighbors?

Which two statements describe the scenario when a router will send out a Type 4 link-state update packet to its neighbors?
A . Type 4 link-state update packets are unicast to every neighbor on regular intervals to keep the database consistent between neighbors.
B . Type 4 link-state update packets are sent when the interface receives a Type 3 Link-State Request packet from a neighbor.
C . Type 4 link-state update packets are multicast to every neighbor on regular intervals to keep the database consistent between neighbors.
D . Type 4 link-state update packets are sent out to the designated router ( DR) or backup designated router ( BDR) when a topology change occurs.

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Answer: BD