液体动力性噪声，由于气体压力突变或流体流动而产生的噪声。
参考答案：对
Someone________the door when she was cooking in the kitchen. [ ]
A. had knocked
B. has knocked
C. knocked at
D. knocked
垂直纸面的匀强磁场区域里，一离子从原点O沿纸面向x轴正方向飞出，其运动轨迹可能是下图中的（ ）
A．
B．
C．
D．
