To investigate friction, a ball is let roll down a ramp. It rolls across the floor and then up a second ramp. This experiment is repeated with the second ramp covered in carpet, sandpaper and felt. Which of these should be a variable in this experiment? A) The angle of the second ramp is a variable. B) The mass of the ball used in the experiment should be a variable C) The height that the ball goes up the second ramp should be a variable. D) The height the ball is let go from for each trial should be a variable.