What makes cellulose nondogestible to humans?

Humans are unable to digest cellulose because the appropriate enzymes to breakdown the beta acetal linkages are lacking. (More on enzyme digestion in a later chapter.) Undigestible cellulose is the fiber which aids in the smooth working of the intestinal tract. ... No vertebrate can digest cellulose directly.

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well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."