Fact 1: Active immunity results when an immune response to a pathogen produces memory cells.

Fact 2: Memory cells persist in the circulation for decades.

Given the two facts above, answer the question "what can persist in the circulation for decades?" with the following options: 
- active immunity
 - density
 - Veins and arteries.
 - oxygen cells
 - bacteria
 - chlorofluorocarbons
 - fur and fat
 - Chemical energy


 

 active immunity        