time to roll down the ramp mass of the ball diameter of the ball kind of surface on the ramp
I gave my students this multiple choice question: Students design an investigation to study the effect of different surfaces on the speed of a ball rolling down a ramp. Which of these is the independent variable?

Only one answer is correct among these 4 choices:
- time to roll down the ramp
- mass of the ball
- diameter of the ball
- kind of surface on the ramp

Could you tell me which one is correct?

kind of surface on the ramp