Answer:
C: Oxygen combines with carbon dioxide.
Step-by-step explanation:
Cellular respiration is a biological process, by which complex molecules, such as glucose is breakdown to produce energy in the form of ATP molecules.
During cellular respiration, glucose molecules are completely oxidized in the presence of oxygen to form ATP molecules, while water and carbon dioxide are formed as byproduct of the reaction.
Hence, oxygen is a reactant and carbon dioxide is a product of cellular respiration that do not combine during this process.
Thus, the correct answer is option (C).