Amino acids are the building blocks that make up proteins. 
        
             
        
        
        
Arrow a should point to muscle tissue, arrow B should point to an organ, arrow c should point to organ system.
        
                    
             
        
        
        
Answer: Base pairs occur when nitrogenous bases make hydrogen bonds with each other. 
Explanation: Each base has a specific partner: guanine with cytosine, adenine with thymine (in DNA), The hydrogen bonds are weak allowing it to "unzip"