Context: Dan and his sister are building ramps to race their toy cars down. Dan notices that the cars go down some of the ramps faster than others. He wonders what factors affect the cars' speed. So, he decides to design an experiment. He has the following supplies available:
two identical toy cars
a wooden ramp three feet long and two feet tall
a cardboard ramp three feet long and two feet tall
Using only these supplies, which question can Dan investigate with an experiment?
A. Do toy cars go faster down the ramp made of wood or the ramp made of cardboard?
B. Do toy cars with plastic wheels go faster down the cardboard ramp than toy cars with metal wheels?
C. Does a big toy car go down the wooden ramp faster than a small toy car?
Select the correct answer from the provided options and reply with the letter associated with it.

A