Correct Option: A

This element is obviously the most important element in the human body. Oxygen atoms are present in water, which is the compound most common in the body, and other compounds that make up tissues. It is also found in the blood and lungs due to respiration. In the human body, the oxygen is absorbed by the blood stream in the lungs, being then transported to the cells where an elaborated change process takes place. Oxygen plays a vital role in the breathing processes and in the metabolism of the living organisms. Probably, the only living cells that do not need oxygen are some anaerobic bacteria that obtain energy from other metabolic processes.