The principle is called CEPHALOCAUDAL PRINCIPLE.
This principle proposed that growth follow a particular pattern in which the head and the upper part of the body grow first before the growth proceeds to other part of the body. 
        
             
        
        
        
A. Heat will not flow because there is no temperature difference between the substances.
        
                    
             
        
        
        
The answer is water. it is found in our bodies (sorry the professor said its technically closest to oxygen because oxygen is most of our body)
        
             
        
        
        
The house fly develops in four stages: egg, larva, pupal, and adult.
        
                    
             
        
        
        
Answer:
So you dont spread the germs along with wearing masks this helps a lot.
Explanation: