You can further understand a text if you ask questions. 
        
             
        
        
        
Answer:
oxygen
Explanation:
oxygen is an essential nutrient which the body needs especially during respiration.
it help the blood cells do provide energy.
without oxygen, there is no gaseous exchange
 
        
                    
             
        
        
        
Answer:
A. Oxygen 
Explanation:
During cellular respiration the cell uses oxygen to break down sugar. When sugar breaks down it produces energy in which your body needs. Blood picks up oxygen and releses carbon dioxide into your lungs. 
Hope this helps!!
 
        
             
        
        
        
The answer is A the central nervous system sends a message to the brain and then peripheral nervous system sends out the message to react. I think hope I helped