How does the sound of the recorder travel through the air?

Sound always travels in waves regardless of whether it goes through a gas, liquid or a solid medium. They move by particles that collide with one another. It's a domino effect as one particle hits another much in the same way that the heat travels as well

Every physical object causes vibrations when it moves in the air. This leads to creation of waves in the air that then continue to travel as a form of sound. Much like the drum example we've mentioned above, our vocal cords also vibrate when we talk. This vibration happens in air, solid mediums and liquid.

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the device is called “ rocket engine”

the answer is b ) square root of 5