The correct answer here is mutualism. The bacteria in the cow's digestive tract serve both themselves and also the cow itself. The bacteria utilize the food particles passing through the cow's intestines as a source of food. It also uses the warmth and moisture of the cow for protection and as a reproductive medium. But the bacteria also help the cow as well. They help the cow break down cellulose, a sugar polymer commonly found in plant tissues for structural support. The bacteria help the cow break down the cellulose for digestion and use as food. Humans do not possess these types of bacteria so cellulose is mostly passed through humans as waste.