Answer: The duodenum, jejunum, and ileum.
Explanation:
The jejunum is roughly 2.5 meters in length, contains plicae circulares (muscular flaps) and villi to absorb the products of digestion.
Once these resources are used up, they cannot be replaced. Humanity would be in trouble because we are dependent on them to supply most of our energy needs.
The amount of energy at each trophic level decreases as it moves through an ecosystem. As little as 10 percent of the energy at any trophic level is transferred to the next level; the rest is lost largely through metabolic processes as heat.