DL/T634.5101-2002规约采用3种帧格式，即固定帧长、可变帧长和（）模式。
A、单字符
B、单字节
C、双字符
D、双字节
参考答案：A
our textbook ,the English spoken in England in the past is very different from ____ spoken at present.
A．Based on; that
B．Bases on; that
C．Base on; one
D．Based on; it
简述井下事故与复杂情况应遵循的原则？
