Which of the following structures is not part of the conducting zone of the respiratory system?

 # Which of the following structures is not part of the conducting zone of the respiratory system?

 A. Trachea

 B. Main bronchi

C. Alveolar duct

D. Pharynx

The correct answer is C. Alveolar duct.

The conducting zone of the respiratory system includes structures that facilitate the movement of air into and out of the lungs but are not directly involved in gas exchange. These structures include the pharynx, trachea, main bronchi, and other bronchi and bronchioles up to the terminal bronchioles. The alveolar duct, however, is part of the respiratory zone, where gas exchange occurs, as it leads to the alveoli.