Did you ___________ a nice trip?[ ]
A. has
B. have
C. having
D. had
答案：B
下列表现对诊断物质依赖没有帮助的是
A．耐受性增加B．渴求C．冲动性使用物质D．带来严重不良后果E．戒断症状
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．Switched Circuits
