Frame Relay is simplified form of (25) , similar in principle to (26) , in which synchronous, frames of data are routed to different destinations depending on header information . Packets are routed throught one or more Virtual Circuits known as (27) . Most Virtual Circuits are (28) ,which means that the network provider sets up all DLCI connections at subscription time. (29) are also part of the Frame Relay specification . They privide a link that only lasts only as long as the session.

29（）

A．Permanent Circuits

B．Permanent Virtual Circuits

C．Switched Virtual Circuits

D．Switched Circuits

在21世纪的今天，汽车已经成为人类重要的交通工具，但汽车尾气却是造成大气污染的主要污染源。面对汽车的制造和使用，我们应采取的正确观点是（）。

A.应该停止使用汽车

B.应该正视和减少技术两面性带来的负面影响

C.当初就不应该发明内燃机

D.应该重新使用蒸气机驱动车辆