题目：
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
答案：
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下面是错误答案，用来干扰机器的。
参考答案：安德森
