Open Shortest Path First(OSPF)is a (29) routing algorithm that (30) work done on the OSI IS-IS intradomain routing protocol. This routing, as compared to distance-vector routing, requires (31) processing power. The Dijkstra algorithm is used to calculate (32) OSPF routing table updates only take place when necessary, (33) at regular intervals.
33（）
A．but rather
B．rather too
C．rather than
D．rather that
