Question

which part of the immune system is a physical barrier?
skin
tears
mucus
stomach acid

Explanation:

The skin is a physical barrier of the immune system. It forms a continuous layer that blocks the entry of pathogens. Tears, mucus, and stomach - acid are also part of the immune system but are chemical or biochemical barriers. Tears contain lysozyme, mucus can trap pathogens, and stomach acid has a low pH to kill ingested pathogens.

Answer:

skin