Natural gas is a NONRENEWABLE source of energy. All the other ones are renewable. 
-Seth 
 
        
                    
             
        
        
        
The answer is Glycolysis. Glycolysis is the first stage of Respiration that happens in the cytoplasm of the cell. In this process, a 6-carbon sugar molecules is broken down to form 2 molecules of 3-carbon pyruvate. The process of Glycolysis generates ATP in the presence of oxygen.
        
             
        
        
        
Continuous cell lines can be maintained through an indefinite number of generation :)