the whistle material to shake. the air inside the whistle to vibrate. the air to be absorbed by the whistle. the air in the whistle to heat up and expand.
I gave my students this multiple choice question: A whistle produces a sound because blowing air in the whistle causes

Only one answer is correct among these 4 choices:
- the whistle material to shake.
- the air inside the whistle to vibrate.
- the air to be absorbed by the whistle.
- the air in the whistle to heat up and expand.

Could you tell me which one is correct?

the air inside the whistle to vibrate.