Alonzo and Meg were testing different surfaces to see which one had the least friction. They predicted that the car, seen above, would travel FASTEST down the ramp when the surface covering had the LEAST friction. Alonzo and Meg used carpet, sandpaper, and glass to cover the ramp. They let the car go, not pushing it, but letting gravity pull the car to the bottom of the ramp. What was/were the controlled variable(s) in the experiment?