21. on first attachment is secondary succession
22. is the degree of variation of life....
genetic, species or ecosystem within a specific area, system or planet
23. the last one is wrong = )
        
             
        
        
        
Answer:
Answers A, B, C, D and E are all correct
Explanation:
Viruses are non living, ultramicroscopic cells, infectious particles with a protein coat surrounding the nucleic acid that replicate inside any life form and infeft them. 
Bacteriophages, like viruses that can also infect the animal cells infected by the bacteria.
 
        
                    
             
        
        
        
Weather condition , temperature , and food ability 
        
             
        
        
        
Answer:
c they let people know what dinosaurs looked like
 
        
             
        
        
        
If I am not mistaken, the answer is branch. For example a branch of the military is the U.S Navy.