Fact 1: Blowing air into a whistle causes air to vibrate inside the whistle.

Fact 2: Sounds are caused by vibrations in the air.

Given the two facts above, answer the question "Blowing what into a whistle causes sound?" with the following options: 
- a lack of noise
 - CFCs
 - CO 2
 - a lack of sound
 - wind
 - air
 - gas
 - no noise


 

      air   