按结构化设计的思想编制应用程序时，最重要的是（ ）

A．贯彻系统设计的结果

B．避免出现系统或逻辑错误

C．具有丰富的程序设计经验

D．必须具有系统的观点

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.

（55）是（）

A. Permanent Circuits

B. Permanent Virtual Circuits

C. Switched Virtual Circuits

D. Switched Circuits