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

消失模铸造的主要缺点是浇注时塑料模气化有异味，对环境有污染，铸件容易出现与泡沫塑料高温热解有关的缺陷，如铸铁件容易产生（）、夹渣等缺陷。