The answer is C- Excretory system. Hope this helps :)
        
                    
             
        
        
        
To preserve and keep them fresh
        
                    
             
        
        
        
D-walls and doors would represent the cell membrane
        
             
        
        
        
Cellular Respiration is the process in which glucose is broken down with the help of Oxygen to get ATP and Carbon Dioxide and Water are the waste products