Final answer:
The correct answer is A. red blood cells. Tears, mucus, and skin are important physical and chemical barriers that prevent pathogen entry.
Step-by-step explanation:
The correct answer is A. red blood cells. Red blood cells, also known as erythrocytes, are not involved in preventing pathogens from entering the human body. Red blood cells primarily function in transporting oxygen to body tissues and removing carbon dioxide.
On the other hand, tears, mucus, and skin are important physical and chemical barriers that prevent pathogen entry. Tears contain lysozyme, an enzyme that can kill bacteria, while mucus traps and rinses away pathogens. The skin acts as a physical barrier and produces antimicrobial substances.