Question

what do saliva, tears, and nasal mucus have in common?
all can be classified as biological barriers.
all contain langerhans cells.
all can destroy bacterial cell walls.
all are part of the complement system.

Explanation:

1. Analyze Option A: Biological barriers include physical (like skin) and chemical (like these secretions) barriers. But saliva, tears, and nasal mucus are more chemical/secretory, not the main "biological barriers" in the typical sense of the term's classification here.
2. Analyze Option B: Langerhans cells are in the skin (epidermis) for immune surveillance, not in saliva, tears, or nasal mucus. So this is incorrect.
3. Analyze Option C: Saliva has lysozyme, tears have lysozyme, and nasal mucus can have antimicrobial components. Lysozyme destroys bacterial cell walls (breaks down peptidoglycan). So this is correct.
4. Analyze Option D: The complement system is a group of plasma proteins in blood, not in saliva, tears, or nasal mucus. So this is incorrect.

Answer:

C. All can destroy bacterial cell walls.