Q: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
A:
active immunity