题目内容：
根据下列题干，回答题：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 Dijkstra algorithm is used to calculate （ 4 ）OSPF routing table updates only take place when necessary，（ 5 ） at regular intervals．
请选择（1）处最佳答案：
参考答案：
答案解析：