That back-and-forth movement also adjusts the muscles in your legs and belly.
那种前后运动也能调节你腿部和腹部的肌肉。
Generally, the reason for such seasonal back-and-forth movement is to seek resources that aren't available within a single area year-round.
一般来说，进行这种季节性往返迁移是为了寻找在一个地区无法全年获得的资源。
Sound is caused by vibrations. A vibration is a back and forth movement.
声音是由振动引起的。振动就是往复运动。
This back-and-forth movement of the disc causes air in front of the receiver to move back and forth in the same way.
这种往返运动使听筒前的空气以同样的方式做往返运动。
This back-and-forth movement of the disc causes air in front of the receiver to move back and forth in the same way.
这种往返运动使听筒前的空气以同样的方式做往返运动。
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