Frame Relay is simplified form of (51) , similar in principle to (52) , 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 (53) . Most Virtual Circuits are (54) , which means that the network provider sets up all DLCI connections at subscription time. (55) are also part of the Frame Relay specification. They privide a link that only lasts only as long as the session.

A．Permanent Circuits

B．Permanent Virtual Circuits

C．Switched Virtual Circuits

D．Switched Circuits

依靠流体（气体或液体）的流动来传递（）的过程或方法称为对流换热。

A、热量

B、质量

C、流量

D、电量