Answer:
Oxygen is absent.
Step-by-step explanation:
Anaerobic respiration can be defined as breakdown of complex glucose molecule, in the absence of oxygen. It provides energy in the form of ATP molecules as it leads to formation of two ATP molecules per glucose molecule. Final product of anaerobic respiration is lactic acid, instead of water and carbon dioxide as in aerobic respiration.
Thus, the correct answer to be filled in the blank is 'oxygen is absent.'