How Would Cellulose-Degrading Bacteria in the Rumen (Stomachs) of a Cow

How would cellulose-degrading bacteria in the rumen (stomachs) of a cow benefit the animal?

A) The bacteria would produce, secrete, and incorporate organic molecules as they obtain energy from the cellulose in grass and replicate. The cow could then digest both the secreted organic molecules AND the bacteria that synthesized them.
B) The bacteria would produce sugars as they digested the cellulose. These sugars would be given off to the cow as a symbiotic relationship in exchange for the protected location of the stomachs of the cow for the bacteria to grow and replicate in.
C) They wouldn't benefit the animal - they would replicate and cause disease in the animal, leading to its death.
D) They wouldn't benefit the animal but they would benefit the other bacteria within the animal. This is why cows must have food sources other than grass in order to survive.
E) They are needed to break down endospores of potential pathogens that are certain to cause illness or even death in the cow if it consumes them when it eats grass.