A X-rays I think this is the answer
        
                    
             
        
        
        
THE ANSWER IS C.) MITOTIC DIVISON
I GOT IT RIGHT ON USATESTPREP...
        
                    
             
        
        
        
 <span>Living things are biotic factors like a dog, fungi and trees
I hope this helps!</span>
        
             
        
        
        
Answer:
I don't know I'm sorry I will tell you another answer asks me 
 
        
                    
             
        
        
        
I only have one reason but hope it helps!
Because without those bases pairing there <span>would be no structure of a DNA.</span>