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．connectionless

B．connection—oriented

C．datagram

D．telegram

简述伏赫（V／H）限制的作用及原理。