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（2）处应选择（）
A.processor
B.device
C.route
D.terminal
参考答案：C
解析：
实现面向连接的服务需要一个虚电路子网，下面看它如何工作。虚电路的主要思想是避免为每一个发送的分组都进行一次新的路由选择。事实上，在连接建立过程中，从源主机到目标主机的路由已经选定，并存储在路由器内部的路由表中。连接上的所有通信都使用这个路由，与电话系统的工作方式完全一样。当连接释放时，虚电路也终止了。在面向连接的服务中，每一个分组都含有一个标识符，用于表示该分组所属的虚电路号。
