You are presented with the question "what can persist in the circulation for decades?" and the following answer choices: 
- active immunity
 - density
 - Veins and arteries.
 - oxygen cells
 - bacteria
 - chlorofluorocarbons
 - fur and fat
 - Chemical energy

Now knowing that active immunity results when an immune response to a pathogen produces memory cells and memory cells persist in the circulation for decades, choose the best answer.


 

 active immunity        