Awindmill converts wind energy into electricity primarily by using mechanical energy, and windmills are machines operated by windpower. Given these facts, what converts wind energy into electricity among the following options:
- clouds
 - mechanical waves
 - windpower operated machines
 - Movement of an air mass
 - solar panels
 - Warm air moving into cold air
 - batteries
 - tectonic plates being pushed together


 

   windpower operated machines      