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.

The idea behind virtual circuits is to avoid having to choose a new（）for every packet sent.

A．processor

B．device

C．route

D．terminal

在一甲醇-水的精馏系统中，馏出液甲醇含量不小于98％，残液中甲醇含量不大于0.05％，入料甲醇含量75％，沸点进料，其入料塔板汽相中甲醇含量85％，其最小回流比是（）。

A、1.3

B、2.3

C、0.833

D、1.769