Question

in order for a physical barrier to be effective in preventing infection, the barrier must __________. a. prevent the substance from entering the body, trap the substance when it first enters the body, or flush the substance out of the body b. destroy the substance before it enters the body, prevent the substance from reproducing, or flush the substance out of the body c. trap the substance in the body, remove the substance from the body, or prevent the substance from reproducing d. flush the substance from the body, transport the substance in the body, or destroy the substance in the body

Explanation:

Physical barriers like skin, mucus membranes etc. work by preventing pathogens (the'substance' here) from entering the body (e.g., skin acts as a cover). Mucus in respiratory tract can trap pathogens (trap the substance when it first enters). Cilia in respiratory tract can help flush out trapped pathogens (flush the substance out). Options B, C and D are incorrect as physical barriers don't destroy pathogens before entry (B - that's more of chemical barrier or immune cells), don't prevent reproduction (C - immune cells like T - cells do that in adaptive immunity) and don't transport pathogens in the body (D - that's not a function of physical barrier).

Answer:

A. prevent the substance from entering the body, trap the substance when it first enters the body, or flush the substance out of the body