While troubleshooting some connectivity issues， you issue the "show ip ospf database" in order toexamine the link state database. Which three of the statements below are true regarding theOSPF link state database？（）

A. Each router has an identical link state database
B. External routes are imported into a separate link state database
C. Synchronization of link state databases is maintained via flooding of LSAs
D. Information in the link state database is used to build a routing table by calculating a shortest-path tree
E. By default， link state databases are refreshed every 10 minutes in the absence of topologychange