Answer:
Adrian's adaptive immune system remembered the antigens from the virus. so he becomes immune to it. 
Explanation:
 
        
             
        
        
        
The answer is B. a nucleus.
Prokaryotic cells do not have a nucleus. They have ribosomes, a cell membrane, a cell wall, and DNA, but lack a nucleus. Prokaryotes also don't have mitochondria or chloroplasts and generally lack organelles.
 
        
                    
             
        
        
        
There are so many examples for that in different areas, like organic germanium compounds experiment carried out in our lab recently.
Here's one link: http://www.alfa-chemistry.com/products/organic-germanium-24.htm