环回地址是用来将分组从( )发送给( )。A．主机 所有其他主机 B．路由器 所

如图所示的滑轮组，小李用750N的拉力将重为1200N的物体在10s内匀速提高2m（不考虑绳重及摩擦）。 求：

（1）拉力做的功

（2）拉力做功的功率

（3）滑轮组的机械效率

（4）若用此滑轮组提升重800N的物体，所需拉力的大小。

在固定岗位工作的保安员应了解电闸、灭火器、消火栓等安全设备的（）

A.所在位置

B.工作原理

C.使用方法

D.性能

E.维修方法

天然橡胶具有很高的耐油性。

For (21) service，we need a virtual-circuit subnet．Let US see how that works．The idea behind virtual circuits is to avoid having to choose a new (22) for every packet sent．Instead， when a connection is established，a route from the (23) machine to the destination machine is chosen as part of the connection setup and stored in tables inside the (24) ．That route is used for all traffic flowing over the connection，exactly the same way that the telephone system works． When the connection is released．the virtual circuit is also tenninated．With connection-oriented service，each packet carries an (25) telling which virtual circuit it belongs to．

A．source

B．route

C．destination

D．host

如何促进知识的保持