Answer:
The letter A is the right answer.
Explanation:
Cellular respiration occurs in the presence of oxygen. This element is essential in the cellular respiration process, since it is responsible for accepting electrons and keeping the electron transport chain functioning.
Cellular respiration occurs after glycolysis, where pyruvate is formed along with 2 molecules of ATP. In the presence of oxygen, pyruvate is oxidized to a two-carbon compound that will react with co-enzyme-A and form NADH and CO2. This process will start the electron transport chain that will result in a protonic gradient that will transform ADP into ATP. The electron receptor in this process is oxygen, which will later be combined with hydrogen molecules to form water.
