当直埋光缆纵剖钢管十分困难时，一般可通过（）而得到解决。

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 internet works differ slightly from the idealized graph model．In an internet work，the goal of the routers to forward (72) to various networks．
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．
Routing protocols use different techniques for assigning (74) to individual networks．Further, each routing protocol forms a metric aggregation in a different way．RIP takes the simplest approach，with all link costs being equal (75) ．Thus it always tries to find the minimum hop route．Valid distances are 1 through 15．This also limits RIP to runing on fairly small networks．

A．10
B．30
C．60
D．180