Which of the following best describes an example of a nonspecific external response to harmful bacteria? a) phagocytes engulf and destroy invading bacteria. b) histamine causes areas invaded by bacteria to swell. c) antibodies recognize bacteria and attack them. d) enzymes in saliva break down bacterial cell walls.

The correct answer is option D.

Explanation:

The outer surface of body defenses is nonspecific, which means they do not attack specific pathogens. Skin is the very first immune system against nonspecific pathogens. Other than mucous membranes also act as a barrier to pathogens and produce chemical defenses.

Saliva also provides a non-specific external response as it contains mucus, antibacterial compounds and various enzymes to target bacteria and other pathogens.

Thus, the correct answer is option D.