变电站站内道路布置除满足运行、检修、消防及设备安装要求外，还应符合带电设备安全间距的规定，下列哪个变电所的主干道路应尽量布置成环形？（）

A.35kV变电站

B.66kV变电站

C.110kV变电站

D.220kV变电站

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