the amount of air in each ball the distance each ball is dropped the time it takes to drop each ball the number of times the ball bounces
I gave my students this multiple choice question: Ben wants to know if the amount of air in a basketball affects how high the basketball bounces. He fills three basketballs with air and drops each one three times. Which part of the experiment should Ben keep the same for each ball?

Only one answer is correct among these 4 choices:
- the amount of air in each ball
- the distance each ball is dropped
- the time it takes to drop each ball
- the number of times the ball bounces

Could you tell me which one is correct?

the distance each ball is dropped