The correct answer is C. sewage discharge.
Even though there might even be no harmful pathogens at all if there isn't any waste, it is correct because it is implied that there is waste such as fecal matter inside of the sewage discharge.
The correct answer is "fiber".
Fiber is actually indigestible food products such as cellulose that aids in the digestion and bowel function. Fiber, first of all, limits the absorption of extra fats therefore preventing hypertriglyceridemia and eventually cerebrovascular and cardiovascular consequences. Fiber also limits glucose absorption controlling type II diabetes. Lastly, fiber also acts as protection in the colon by increasing the transit times of potentially carcinogenic substances found in food, therefore preventing colon cancer.
The ecosystem requires a constant input of energy because energy cannot be stored. The enrgy always gets converted for supporting life in the ecosystem. Some amount of energy is also lost during this conversion. So if there is not a constant supply of energy, then the ecosystem will ultimately breakdown. In case of humans, the energy is received by consuming food. The energy of the food geets converted to the enrgy that helps a human being to complete his daily chores. So energy is being lost at every step. To regain this energy the humans have to eat food regularly. This way the humans need a constant supply of energy to survive. It is the same in the case of all living things in the ecosystem.
