Here's a problem to solve: A student plans an investigation to calculate the speeds of different balls as they roll down a ramp. Which piece of information is least necessary for the student to record?

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

- A: the size of each ball
 
- B: the thickness of the ramp
 
- C: the length of the ramp
 
- D: the time it takes the ball to travel
 