The famous scientist Galileo Galilei did several experiments with sloping planes, which he rolled metal balls down so that he could study motion. By changing the slope, he could study how the speed at which the ball rolled was affected. What was one thing Galileo would have needed to control himself to make sure his experiment gave him useful information?

A. the speed of the ball
B. the slope of the plane
C. the pull of Earth's gravity
D. the material the ball was made of