Charlie tested three types of toy cars. He placed the cars on the same starting line on the same inclined plane. He released the three cars at the same time. He measured how far the cars traveled when rolling down the plane, to see which type of car rolled furthest. What is the independent variable in this experiment?

A. the ruler used to measure distance

B. grade of inclined plane

C. type of car

D. Charlie

PLEASE ANSWERR