A. Immune system 
It helps our bodies fight everything that comes in our path.
        
                    
             
        
        
        
there is mostly billions of amount of water from raindrops each day i think. 
If You want a EXACT number i'll say, 1,100.
 
        
             
        
        
        
Metamorphosis through which the insect develops by four distinct stages: egg, larva, pupa, adult. A type of metamorphosis in which an organism's transformation is so dramatic that it is difficult to recognize the relationship between the larva and adult form.
Option 4) Metamorphosis