If OSPF is used in a multi-area OSPF network, which of the following statements regarding...

Manual route summarization can only be done on autonomous system border routers.

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.

Manual route summarization is done on the ABRs. By default, automatic summarization is done; however, it may not be optimal for all networks.

Manual route summarization is done on the ABRs. By default, no route summarization is done and all routes are advertised to all areas.