Fact 1: Uneven heating of the Earth 's surface cause wind.

Fact 2: Wind is produced as the sun heats the air.

Given the two facts above, answer the question "what causes wind?" with the following options: 
- speed
 - the sun
 - energy
 - supersonic
 - Energy.
 - erosion
 - a chime
 - messengers


 

  the sun       