You are presented with the question "what is fan speed?" and the following answer choices: 
- energy barrier
 - kinetic energy
 - fixed
 - cycles
 - Winter
 - static
 - solid
 - adjustable

Now knowing that the vanes rotating in an electric fan causes air to move and output of the fan is varied by means of a series of adjustable vanes upstream of the impeller, choose the best answer.


 

        adjustable 