Students in science class were testing different surfaces to see which one had the lowest coefficient of friction. One student group predicted that the car, seen above, would travel FASTEST down the ramp when the surface covering had the LOWEST coefficient of friction. The students used carpet, sandpaper, and glass to cover the ramp for each trial. They let the car go, not pushing it, but letting gravity pull the car to the bottom of the ramp. Consider the students' experimental design. To be a valid experiment, there can be only one independent variable. What variables in this experiment were controlled? A Number of trials conducted on each surface. ◄× B √x C D The texture of each ramp surface used. Ramp height, angle, car used, and force. The ramp surfaces used in each trial.​