59.Cows are able to survive on a diet consisting almost entirely of cellulose because(A) cows are autotrophic; (B) cows can manufacture all 20 amino acids out of sugars in the liver; (C) unlike humans, the saliva the cow produces has enzymes capable of digesting cellulose; (D) cows have cellulose-digesting, symbiotic microorganisms in their rumens.