Here's a problem to solve: 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?

Among the 4 following options, which is the correct answer?

- A: time to roll down the ramp
 
- B: mass of the ball
 
- C: diameter of the ball
 
- D: kind of surface on the ramp
 