Question

which of the following is an example of mutualism?

A bacteria live in root nodules of a plant, processing nitrogen into ammonia for the plan 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