Cellulose could provide an abundant and cheap form of glucose for humans. why is cellulose not a source of nutrients for humans? humans, and most vertebrates, lack the enzyme cellulase. human gut bacteria break down cellulose in the large intestine, after nutrient absorption occurs. animals use starch and glycogen as an energy source, whereas plants use cellulose for energy. humans cannot absorb the hydrolyzed form of cellulose. vertebrate enzymes hydrolyze (α 1–4) glucose linkages, but not glucose in the β configuration.

Humans, and most vertebrates, lack the enzyme cellulase.

Explanation:

Cellulose is a polysaccharide and it is made up of glucose however we the humans and other vertebrates lack the enzyme cellulase which is required to break down the cellulose molecule.

Some Animals such as cows, and termites have  bacteria in the intestinal tract which can digest the cellulose as it has cellulase enzyme.

Thus due to symbiotic association with bacteria a few animals may digest cellulose indirectly.

the correct answer is:

a. formation of precipitation