按叶轮的数目分为单级离心泵和双级离心泵（）

In order not to miss his flight, he ran ______ the street, jumped______ the fence and dashed into

the waiting hall, ______ all about his own safety.    [      ]

A. across; over; forgetting  

B. through; across; forgot  

C. over; over; forgetting  

D. across; over; forgot

林可霉素类可能发生的最严重的不良反应（）。

A.过敏性休克

B.肾功能损害

C.永久性耳聋

D.胆汁淤积性黄疸

E.伪膜性肠炎

简述水稻品种南北相互引种后预期的效应。