空气滤清器或滤芯的上游和下游规定测压点所测得的静压差是（）
A.绝对压力
B.压力降
C.相对压力
D.压力值
参考答案：B
Wang Xiaobo was a quite special writer. His books are well worth ______. [ ]
A. reading
B. being reading
C. seeing
D. being seen
超声波归属于何种波（）。
A.光波
B.电磁波
C.机械波
D.微波
E.粒子波
