正确处理人与自然的关系，不仅需要人与自然的道德规范，还需要法律保障。在我国，环境保护的基本法是[ ]

A．《中华人民共和国宪法》

B．《中华人民共和国矿产资源法》

C．《中华人民共和国环境保护法》

D．《中华人民共和国大气污染防治法》

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.

67（）

A．X.21

B．X.25

C．X.28

D．X.29