—Mum, I can't see any point in studying hard.

—Come on. dear,Years of hard study will surely        in your future career.

A．make sure

B．pay off

C．bring back

D．pay back

洁净室新风量的确定不需要考虑的因素是( )。

A．作业人员健康所需新鲜空气量

B．维持静压差所需补充风量

C．补充各排风系统的排风所需新风量

D．洁净室的送风量及气流组织