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

读我国两个区域图，回答1—2题。

1、某日，甲地比乙地的白昼长50分钟，则甲地比乙地[ ]

A、日出早25分钟

B、可能同时日出

C、日落迟25分钟

D、日落迟45分钟

2、丙附近湖泊面积不断缩小，要及时获取湖泊面积变化情况，最快捷的途径是[ ]

A、RS技术

B、GPS技术

C、GIS技术

D、实地考察