Here's a problem to solve: Which statement is not a reason why trees would be growing at different heights in a forest?

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

- A: Some require less sunlight than others.
 
- B: The trees were planted in different years.
 
- C: Birds are building nests in the shorter trees.
 
- D: The taller trees only have leaves on the top branches.
 