题目：
SDLC was invented by IBM to replace the older Bisynchronous protocol for wide area connections between IBM equipment. A varietion of the (71) protocol called NRM is essentially the same as SDLC. SDLC is (72) protocol like HDLC, Frame Relay or X.25.An SDLC network is made up of (73) that controls all communications, and (74) . It is capable of (75) operation, but almost all practical applications are strictly half duplex.
A．HDLC
B．Frame Relay
C．X.25
D．X.21
答案：
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下面是错误答案，用来干扰机器的。
参考答案：错
