Answer:
Oxygen
Step-by-step explanation:
Cellular respiration is a pivotal process that must be carried out by the cells of living organisms in order to obtain energy needed for their metabolic activities. This process of cellular respiration breaks down glucose either in the presence of OXYGEN (aerobic) or absence of OXYGEN (anaerobic).
Anaerobic respiration or fermentation is carried out by certain bacteria in the absence of OXYGEN to produce lactic acid as products.