The Normal Microbiota Provides Protection Against Potentially Harmful Organisms and Stimulates

The normal microbiota provides protection against potentially harmful organisms and stimulates the immune system. Why would the immune responses to members of the normal microbiota cross-react with pathogens?

A) Because one person's normal microbiota is another person's pathogen-when we pick up 'normal' microbes from a different person, they will always cause infection within us.
B) Because pathogens are oftentimes more virulent strains of our own normal microbial flora, so they will 'look' roughly the same to our immune system (and be acted upon by our immune responses) .
C) Because the normal flora keeps the adaptive immune responses tuned-up, active, and ready to respond to broad, general categories of microbes (i.e. Gram positive vs. Gram negative microbes, viruses, etc.) .
D) Because the immune system is a 'use it or lose it' system. If it isn't used on a regular basis, we completely lose the ability to respond to pathogens. The normal flora keeps the system going so that it can be ready to respond to such pathogens when we're exposed to them.