The correct option is D Alveolar sacs
The human respiratory system can be structurally divided into two main zones - the conducting zone and the respiratory zone. The former includes structures that function as a passageway that carries air into and out of the lungs. These structures are the nasal cavity, the oral cavity, pharynx, larynx, trachea, bronchi, bronchioles and the terminal bronchioles. The respiratory zone includes structures involved in the exchange of gases between the blood and the air. These are the lungs, respiratory bronchioles, alveolar ducts and alveolar sacs.