Final answer:
The primary function of the upper passageways of the lungs is pathogen filtration, where mucus traps pathogens and debris, and cilia move the mucus toward the throat.
Step-by-step explanation:
The primary function of the upper passageways of the lungs, which include the nasal cavity, pharynx, and larynx, is pathogen filtration. These structures are part of the conducting zone and are lined with pseudostratified ciliated columnar epithelium with goblet cells. The mucus produced by the goblet cells traps pathogens and debris, while the cilia move the mucus toward the throat to be swallowed, thus filtering the air before it reaches the lower respiratory tract. These upper passageways play a vital role in defending the respiratory system against inhalation of potentially harmful substances.
Therefore, the correct answer to the student's question is: B. Pathogen filtration. The upper passageways do not produce oxygen, circulate blood, or eliminate carbon dioxide; these functions occur deeper in the lungs, particularly in the alveoli where gas exchange takes place.