Order the following barriers to pathogens in terms of their functioning in the body's defense system: A. Skin oils create an acidic environment B. Mucus traps pathogens C. Skin serves as a physical barrier D. Sebum kills pathogens

The skin’s outer layer is the first line of defense, stopping germs before they enter. Next, the oils it produces make the surface acidic, which keeps many microbes from growing. Those same oils, called sebum, also directly kill some pathogens that slip through. Finally, mucus in places like the nose and throat catches and holds any remaining germs so they can be swallowed or expelled. For example, a splashed spray of bacteria is first stopped by the skin, then the acidic oils weaken it, the sebum destroys it, and any that get past are trapped in mucus.