Question: Which relationship is mutualistic?

  Options: 
 A. an insect that lives and feeds on the body of an alligator
 B. an ant that lives on a plant and defends the plant from other insects
 C. a bird that migrates to follow the movements of the butterflies that it eats
 D. a deer that eats one kind of plant, which allows another kind of plant to grow in its place
Answer: B