Which of the following is an example of mutualism?

a. bacteria live in root nodules of a plant, processing nitrogen into ammonia for the plant and receiving protection and a steady food supply in return.

b. moss benefits from exposure to light and moisture as it grows on a tree trunk that is not affected by its presence.

c. fleas live in the fur of a dog where they periodically pierce the dogs skin and feed on its blood.

d. cowbirds lay their eggs in the nests of other birds that are often tricked into rearing the cowbird chicks instead of their biological chicks.