Answer:
With a compromised immune system, people have a far more difficult time fighting off disease. Due to this, their bodies are more vulnerable to getting such diseases and cannot create the antibodies needed to fight the disease off. 
 
        
                    
             
        
        
        
Answer: Option B) Antigens
Blood typing is based on the presence of proteins known as antigens on the outer surface of the red blood cell plasma membrane.
Explanation:
Blood groups A, B, AB, and O are determined based on the antigen-antibody reactions between donor and recipient bloods.
For instance, 
- Blood type A has antigen A on its plasma membrane 
- Blood type B has antigen B on its plasma membrane
- Blood type AB has both antigen A and antigen B on its plasma membrane
- while blood type O has neither antigen A nor B on its plasma membrane
So, antigens is the answer 
 
        
             
        
        
        
Evolution by natural selection.  
survival of the fittest=natural selection
 
        
             
        
        
        
The confederation has succeeded in establishing a new country. However, the residents of each state still tended to think of themselves as citizens of their particular state rather than as Americans. Under the Articles, many of the states continued to have only limited contact with each other. This made it difficult for them to agree on the common interests and goals for the government.
 
        
             
        
        
        
The element that is most common in Vitamin D is CARBON.