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:

Saliva, tears, and nasal mucus contain lysozyme. Lysozyme can break down the peptidoglycan in bacterial cell walls, destroying them. Biological barriers are physical (like skin), Langerhans cells are in skin, and they're not part of the complement system.

Answer:

All can destroy bacterial cell walls.