单选题 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 ischosen as part of the connection setup and stored in tables inside the ______. That route is used forall 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-orientedservice, each packet carries an ______ telling which virtual circuit it belongs to.