Squamous cells enable gas diffusion in alveoli

Steps:

• Identify epithelial types in respiratory tract: trachea and bronchi lined by pseudostratified ciliated columnar epithelium for mucus clearance.
• Recognize alveoli require thin barrier for O2 and CO2 exchange.
• Recall simple squamous epithelium is flat and thin, matching alveoli structure.
• Match question to gas exchange site: alveoli.

Why D is correct:

• Simple squamous epithelial cells line alveoli, forming a thin barrier that facilitates rapid gas diffusion per Fick's law of diffusion.

Why the others are wrong:

• A: Trachea and bronchus have pseudostratified ciliated columnar epithelium, not squamous.
• B: Trachea and bronchus feature columnar cells for protection and ciliary action.
• C: Trachea is lined with pseudostratified ciliated columnar epithelium to trap particles.

Final answer: D