Not All Parts of the Food That Humans Ingest Are

Not all parts of the food that humans ingest are used by our bodies. Fiber consisting mostly of cellulose, for instance, is excreted without providing nutrition. Which of the following statements explains why fiber is not used as a nutrient in humans?

A) The human gut is unable to break it down; thus it cannot be absorbed across the intestinal cells' membranes.
B) Fiber is an amino acid but not one of the essential amino acids, so we do not need to absorb it as a nutrient.
C) The fiber content of human foods is so low that it is not worthwhile for our bodies to expend energy to extract its nutrients.
D) The fiber is digested into such small molecules that they become nutritionally insignificant.