“上北下南，左西右东”的看地图的方法，适应的情况是

A．所有有经纬网的地图

B．不带经纬网和指向标的地图

C．带有指向标的地图

D．所有地图

For (1) 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  (2) for every packet sent. Instead, when a connection is established, a route from the  (3) machine to the destination machine is chosen as part of the connection setup and stored in tables inside the (4) .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 terminateD. With connection-oriented service, each packet carries an  (5) telling which virtual circuit it belongs to.

空白（5）处应选择（）

A.address

B.identifier

C.interface

D.element