下列运动中属于机械振动的是 [ ]

A．树枝在风的作用下运动

B．竖直向上抛出的物体的运动

C．说话时声带的运动

D．爆炸声引起窗扇的运动

Everyone is here, __________?[ ]

A. aren't they

B. are they

C. is everyone

D. isn't he