使用安全柜时，操作前后均开送风机送风净化（）
A.lmin
B.2min
C.5min
D.10min
E.20min
参考答案：D
With good reasons he is called a fool.
A．With good reasons is he called a fool.
B．He is called fool with a good reason.
C．With good reasons he is called fool.
D．He is called fool with good reasons.
什么是载波通信？
