Students studying potential and kinetic energy designed this experimental set-up. During the initial trial, a ball was rolled down a ramp fro the spot marked "start". The ball rolled down the ramp and then back up the other side, to the spot marked "finish", represented by letter B. The students want to determine the starting point so that the ball will rise up to the point marked by letter A. They release the ball starts at the spot marked "New start" and the ball rolls up the other side to letter A. This is a controlled experiment. All BUT ONE factor is controlled in this experiment. What is that factor?

A. ball used in trials

B. distance between ramps

C. starting point of release

D. angles of incline for ramps ***

Can someone check this?