关于嵌体的适应证下述各项中哪一项能成立

A．只用于后牙牙体缺损

B．只用于前牙牙体缺损

C．可用于前、后牙缺损

D．可常规用于固定桥冠外固位体

E．以上均正确

Frame Relay is simplified form of (71) ,similar in principle to (72) , 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 (73) . Most Virtual Circuits are (74) ,which means that the network provider sets up all DLCI connections at subscription time. (75) 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．Switching Circuits