Cilia's primary role in respiratory defense

Steps:

• Recall that cilia are hair-like structures lining the respiratory tract in the gas exchange system.
• Understand their movement: cilia beat in coordinated waves to propel substances upward.
• Link to function: this motion clears the airways by transporting mucus containing trapped particles.
• Eliminate distractors: match options to known roles of other structures like goblet cells or epithelium.

Why B is correct:

• Cilia function as motile projections that generate a mucociliary escalator, defined as the mechanism sweeping mucus from lungs to throat for expulsion.

Why the others are wrong:

• A: Surface area increase is due to alveoli, not cilia.
• C: Mucus production occurs in goblet cells, not cilia.
• D: Dust and pathogen trapping happens in mucus, not by cilia.

Final answer: B