Zhengzhou was attacked by such a terrible sandstorm ________ few residents had ever experienced before.

A．which

B．as

C．where

D．that

护士挤压胸腔引流管的目的是（）

A.防止引流管打折

B.重建胸腔负压

C.保持引流管通畅

D.防止引流液倒流

E.预防感染