当a>1或a<0时，x>1/a；当a=1时，空集;当0<a<1时，x<1/a；当a=0时，R
无
当单向流洁净工作台设有空气幕时，操作区初始平均风速为（）m/s。
A、1.5~2.0
B、0.5~1.0
C、0.3~0.4
D、0.4~0.5
That car accident _________all his life. After that he was limited to a wheelchair.
A．damaged
B．destroyed
C．harmed
D．ruined
