In computational complexity theory, a problem refers to the abstract question to be solved. In contrast, an instance of this problem is a rather concrete utterance, which can serve as the input for a decision problem. For example, consider the problem of primality testing.
Oxygen plays a vital role in both the aerobic and anaerobic metabolic systems. While oxygen is required as a direct energy source in the aerobic system, it is also essential in clearing the blood of excess lactic acid buildup in the anaerobic system.
   Anaerobic respiration is the process by which non-oxygen dependent  organisms breath. It is mostly commonly used in the process of  fermentation.    Anaerobic respiration is respiration that uses electron acceptors  other than oxygen.Anaerobic respiration can be found in protozoans  and bacteria. Anaerobic respiration is a type of cellular respiration. It occurs  when oxygen is not available or is available in very low  quantities.It is also a respiration in the absence of oxygen.Oxygen is not used as the final electron acceptor.  Anaerobic respiration is the process by which non-oxygen dependent  organisms breath. It is mostly commonly used in the process of  fermentation.    Anaerobic respiration is respiration that uses electron acceptors  other than oxygen.