Here's a problem to solve: In the winter, the fur of the arctic fox is white. In summer, the fur darkens to a reddish brown. What most likely causes the fur of the fox to change color?

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

- A: the amount of sunlight present
 
- B: the fox's habitat
 
- C: the fox's genes
 
- D: the fox's age
 