In the following essay, each blank has four choices. Choose the most suitable one from the four choices and write down in the answer sheet.

Open Shortest Path First (OSPF) is a (1) routing algorithm that (2) work done on the OSI IS-IS intradomain routing protocol. This routing, as compared to distance-vector routing, requires (3) processing power. The (4) algorithm is used to calculate routes. OSPF routing table updates only take place when necessary, (5) at regular intervals.

1（）

A．distance-vector

B．link-state

C．flow-based

D．selective flooding