Answer:
B. So you can always see the entire object at the higher magnification
Explanation:
A microscope is used to view objects smaller than the naked or unaided eyes can see. The objective lens, in conjunction with the occular or eye lens, is used to achieve this purpose. The objective lens are of different magnifications as follows: 10X, 40X, 100X etc. 
However, as the magnification increases i.e. higher objective, the object in the field of view becomes smaller and less focused. Hence, the object in the field of view must first be centered at a lower magnification/objective lens before changing to a higher one in order for the entire object to be seen at a higher magnification. 
 
        
             
        
        
        
Answer:
B: The virus has a long period of latency
 
        
             
        
        
        
The answer is D - cellular respiration <span />
        
                    
             
        
        
        
Answer:
Mass
Explanation:
I'm not for sure but I think its Mass. Please let me know if that was right or wrong.