What is the terminal site in the respiratory system where gas exchange takes place

The human respiratory system is divided into a conducting zone and a respiratory zone based on functional capacity. The conducting zone filters and warms air, while the respiratory zone facilitates the diffusion of gases. The blood-air barrier is an extremely thin membranous structure optimized for the rapid movement of oxygen and carbon dioxide. Rationale: A. The trachea and larynx are superior structures located within the conducting zone of the airway. Their primary roles involve air conduction, protection of the lower airway, and vocalization. Their thick walls and cartilaginous rings prevent the passive diffusion of gases across their epithelial surfaces. B. Alveoli are the specialized, microscopic air sacs that constitute the terminal site of the respiratory tree. They are lined with Type 1 pneumocytes, providing a massive surface area for gas exchange. Capillary networks surround these structures to allow oxygen to enter the blood and carbon dioxide to exit. C. Secondary bronchi are middle-tier branches of the bronchial tree that deliver air to specific lobes of the lungs. Although they possess smooth muscle and cartilage, they lack the specialized epithelium required for gas diffusion. They are part of the anatomical dead space where no exchange occurs. D. Primary bronchi are the first major branches off the trachea leading into the right and left lungs. They serve as conduits for bulk air movement into the pulmonary parenchyma. Because they are thick-walled, they cannot facilitate the molecular transfer of oxygen into the pulmonary capillary blood flow.