—Excuse me! Would you mind not smoking here?

—_______. It's a non-smoking area.

A. Yes.

B. Sorry. I'll put it out at once.

C. That's all right.

在UNIX系统中，一个命令的执行会花费一些时间，为了避免等待，可采用( )。

A．后台命令

B．输入重定向

C．输出重定向

D．管道命令