—Where did you find the professor who made the speech yesterday？

—It was in the hall ______ the students often have a meeting..

A．where

B．which

C．that

D．when

频谱多普勒技术检测多普勒血流时，下列哪项做法无意义()

A．屏住呼吸

B．调节滤波

C．速度标尺

D．取样容积大小

E．超声入射角校正