起重机沿竖直方向匀速吊起重为G的物体，在这一过程中，物体的 [ ]

A.动能增加，重力势能减小

B.动能不变，重力势能增加

C.动能减小，重力势能增加

D.动能减小，重力势能不变

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 ___25___ algorithm. Routing protocols in intemetworks differ slightly from the idealized graph model. In an internetwork, the goal of the routers to forward packets to various ___26___.

Routers running RIP send their advertisement about cost every ___27___ 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 ___28___. Thus it always tries to fend the minimum hop route. Valid distances are 1 through ___29___. This also limits RIP to running on fairly small networks.

27（）

A.10 

B.30 

C.60 

D.180