Answer: a. nasal cavity
Explanation:
The nasal cavity is a external opening of the respiratory tract. This involves two canal like cavities through which air enters the respiratory tract and moves out of the body. These are the two hollow spaces within the nose each of which consists of mucus and lined up with the hairs.
The primary function of the two nasal cavities is to moisturize, warm and filter the air entering the respiratory tract. These cavities filter out environmental contaminants like dust, pollen, molds and others.
