Which type of barriers under innate immunity

To which type of barriers under innate immunity, do the saliva in the mouth and the tears from the eyes, belong?

(1) Physical barriers
(2) Cytokine barriers
(3) Cellular barriers
(4) Physiological barriers

Sol. Answer (4)
Acid in stomach, saliva in mouth, tears from eyes all prevent microbial growth. They are
part of innate immunity and grouped under physiological barriers.