The appropriate answer is C. Fiber. Fiber is the common name for the polysaccharide cellulose. Cellulose is the main component of plant cell walls. Humans are not able to digest this substance so it passes through the digestive system. A relatively high fibre diet is good for the digestive system.
Ruminants such as cattle and goats are able to digest cellulose with the aid of specialized bacteria and a four chambered stomach.
