Here's a problem to solve: Trees need oxygen. Roots close to the surface of the ground take in the oxygen the tree needs. Which organisms help trees get oxygen?

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

- A: woodpeckers making holes in the tree
 
- B: earthworms making holes in the ground near the tree
 
- C: mushrooms growing at the base of the tree
 
- D: squirrels eating walnuts on the ground near the tree
 