Here's a problem to solve: Three baseball players investigate to see who can throw a baseball the farthest. They mark off an area on the playground for their investigation. Which should they keep constant?

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

- A: Height of players
 
- B: Color of the baseballs thrown
 
- C: Order in which the players throw
 
- D: Spot from which players throw
 