The oxygen atoms i believe is the answer  
        
             
        
        
        
HIV is the only disease listed here to be sexually transmitted.
-Tiara
        
                    
             
        
        
        
The most appropriate answer would be Biotechnology. Biotechnology has provided the ability to regrow tissue for knee or hip joints.  It uses biological systems that are believed to help the development of products that is reserved for specific use.