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

Fact 2: Referee's whistle blows.

Given the two facts above, answer the question "what can blow air?" with the following options: 
- wind
 - gravity
 - density
 - volume
 - humans
 - heat energy
 - some mammals
 - referees


 

        referees 