Witch step of cellular respiration will not take place in the absence of oxygen

The answer is; glycolysis

This process converts glucose molecule to pyruvate. It is an oxygen-independent pathway, unlike the Krebs cycle. Glycolysis occurs in the cell cytoplasm while the Krebs cycle (aerobic pathway) occurs in the mitochondria. In the presence of oxygen, the product of glycolysis, i.e pyruvate, is fed to the Krebs cycle.  If oxygen is unavailable the pyruvate is converted to lactate.

the way that the mass of carbon can be conserved through cellular respiration is through a process/law called the law of conservation of mass which even tho it undergoes a process, mass is not be destroyed or changed

i think it is dna, i'm not totally sure but i think it might be.