Answer:
I feel that this is a two part answer
Explanation:
I think that most nosebleeds will stop on their own after a fashion (D) but depending on the severity of the nose bleed then they should C- seek medical attention. 
 
        
                    
             
        
        
        
Something about a drug transplant
 
        
                    
             
        
        
        
Endorphins reducing stress levels
        
                    
             
        
        
        
Experts in trauma and emergency care center call the first hour after serious injury as the most critical in determining chances for survival
        
             
        
        
        
I believe it’s carbon dioxide, because you use your respiratory system to breathe in oxygen, and oxygen goes through your circulatory system. And you breathe out CO2.