Why Can Cows Digest Grass and Humans Cannot

Why can cows digest grass and humans cannot?

A) The human stomach contains a peptide that binds grass into an indigestible mass.
B) Grass contains a lipid that is poisonous to humans.
C) Cows have an enzyme that separates the glucose molecule from the cellulose polymer in grass.
D) Cows have several stomachs in their digestive system.
E) Humans can digest grass but choose not to grow it for food.