Here's a problem to solve: 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?

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

- A: the amount of air in each ball
 
- B: the distance each ball is dropped
 
- C: the time it takes to drop each ball
 
- D: the number of times the ball bounces
 