Part A.
For trial 1 and trial 2 the velocity of the car is constant. Since the velocity is constant there is no acceleration. So, we can conclude the acceleration is equal to zero.
Part B.
Depending on the material of the floor, it affects the movement of the car. This is associated with the coefficient of friction. The higher the coefficient of friction, the greater the opposition to movement. According to this the coefficient of friction for the carpet is higher than the coefficient of friction for tile and wood.
Part C.
For the interval [8,10] in the trial 3, the velocity becomes zero, since the distance isn't changing at all. This happened because of the high coefficient of friction of the carpet, which caused the car to stop.
Part D
In order to the students have more confidence in their result, they can change the following:
1. Repeat the experiment several times on the same surface
2. Have a longer length on each surface