无损检测技术的第一阶段使用无损探伤用符号（）。

A、NDI

B、NDT

C、NDE

D、NDB

Frame Relay is simplified form of (66) , similar in principle to (67) , 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 (68) . Most Virtual Circuits are (69) , which means that the network provider sets up all DLCI connections at subscription time. (70) are also part of the Frame Relay specification. They privide a link that only lasts only as long as the session.

70（）

A．Permanent Circuits

B．Permanent Virtual Circuits

C．Switched Virtual Circuits

D．Switched Circuits