机械设备种类繁多，叉车、铲土运输机械、压实机械、混凝土机械等属于( )机械。

A．工程
B．重型矿山
C．基础
D．化工通用

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.

66（）

A．Datagram Switching

B．Datagram Routing

C．Packet Switching

D．packet Routing