woodpeckers making holes in the tree earthworms making holes in the ground near the tree mushrooms growing at the base of the tree squirrels eating walnuts on the ground near the tree
I gave my students this multiple choice question: Trees need oxygen. Roots close to the surface of the ground take in the oxygen the tree needs. Which organisms help trees get oxygen?

Only one answer is correct among these 4 choices:
- woodpeckers making holes in the tree
- earthworms making holes in the ground near the tree
- mushrooms growing at the base of the tree
- squirrels eating walnuts on the ground near the tree

Could you tell me which one is correct?

earthworms making holes in the ground near the tree