Which example illustrates commensalism and not mutualism?

a) hippo dung is eaten by a large species of catfish native to african lakes. the catfish typically follow the hippo herds.

b) cattle have protozoa living in their gut that digest cellulose. the cows are able to feed on grass, while the protozoa are sheltered inside the cow.

c) oxpeckers remove parasitic fly larvae from the skin of buffalo. in exchange, they are protected from predatory cats, who will not challenge a buffalo.

d) ants 'farm' aphid insects like cattle. aphids suck the sap out of trees, which ants also feed on. in turn, the ants sting predators that attack the aphids.