One of the most widely used routing protocols in IP networks is the Routing Information Protocol (RIP). RIP is the canonical example of a routing protocol built on the______(71) algorithm. Routing protocols in internetworks differ slightly from the idealized graph model. In an internetwork, the goal of the routers to forward packets to various_______ (72). Routers running RIP send their advertisement about cost every________(73) seconds. A router also sends an update message whenever an update from another router causes it to change its routing table. It is possible to use a range of different metrics or costs for the links in a routing protocol. RIP takes the simplest approach, with all link costs being equal _______(74). Thus it always tries to find the minimum hop route. Valid distances are 1 through_________(75) ,This also limits RIP to running on fairly small networks.

空白处____（73）应选择（）

A.10 

B.30 

C.60 

D.180