气刨结束时应先断弧，再关闭压缩空气阀门。( )
参考答案：对
------ Who is that man over there? _______ it be our teacher?
------ No, it _______ not be our teacher. It _______ be Mr. Zhang.
A．can ; can; must
B．must; must; can
C．can; must; must
D．can; must; can
固定式轴流泵只能采用（）
A.变径调节
B.变角调节
C.变速调节
D.变阀调节
