For（）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（）for every packet sent. Instead, when a connection is established, a route from the（）machine to the destination machine is chosen as part of the connection setup and stored in tables inside the（）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（）telling which virtual circuit it belongs to.
With connection-oriented service, each packet carries an（）telling which virtual circuit it belongs to.
A．address
B．identifier
C．interface
D．element
参考答案：B
解析：
题目翻译过来的大概意思是：为了建立一个面向(连接的)服务，我们需要一个虚拟的电路网络。让我们看看它是怎么工作的。虚电路的思想是避免为每个要传送的包建立一条链路，取而代之，当一个连接一旦确定，从(源主机)到目的主机的一条路径就会被选中，然后保存在(路由器)中。这条路径就是以后的包的路径，很像是电话系统。在面向连接的服务中，每个包都带着一些(标识信息)来显示它属于哪条虚电路。