(Topic 1) If OSPF is used in a multi-area OSPF network, which of the following statements regarding route summarization is true?

(Topic 1) If OSPF is used in a multi-area OSPF network, which of the following statements regarding route summarization is true?
A . Manual route summarization can only be done on autonomous system border routers.
B . Manual route summarization must be done on all the backbone routers to be effective. Manual route summarization is optional on the routers in non-backbone areas.
C . Manual route summarization is done on the ABRs. By default, automatic summarization is done; however, it may not be optimal for all networks.
D . Manual route summarization is done on the ABRs. By default, no route summarization is done and all routes are advertised to all areas.

Answer: D