Of all your systems, the integumentary, respiratory, and digestive most regularly experience infection from microorganisms. Which answer best describes why that may be the case?

• A. All three systems are in regular contact with substances from outside the body that may contain bacteria.
• B. All three systems are closely linked to the immune system, meaning they protect against microorganisms.
• C. All three systems have limited blood supply, meaning the body has difficulty fighting infection in those places. 
• D. All three systems function to transfer materials from outside of the body to inside the body, giving microorganisms and entry point.