We must keep our room clean, for dirt and disease go______, you know.

A．hand in hand

B．step by step

C．from time ti time

D．one zfter another

正常主动脉瓣口血流多普勒频谱的特点是（）。

A.频谱幅度比肺动脉口高

B.频谱为双峰型

C.舒张期出现

D.频谱幅度比肺动脉口低

E.频谱为双向型