(A)
The comparison is unfair because the height where each object is dropped is not the same, so he can't actually compare each bounce because the initial height is different, therefore, his results will be biased. Another reason is the material of the floor, it also should be the same for each case.
(B)
The changes the student can make are the type of floor and the height ball dropped. If the student uses the same type of floor and the same initial height, the experiment won't be biased.