Here's a problem to solve: A squirrel dropped an acorn from a branch of an oak tree. Which of these will change as the acorn falls from the tree to the ground?

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

- A: the mass of the acorn
 
- B: the force on the acorn
 
- C: the density of the acorn
 
- D: the velocity of the acorn
 