What Is the Primary Anatomical Difference That Marks the Dividing

Multiple Choice

What is the primary anatomical difference that marks the dividing line between the conducting zone and the respiratory zone of the respiratory tract?

A) the absence of cartilage
B) the presence of macrophages
C) the absence of goblet cells
D) the presence of smooth muscle and the absence of cartilage
E) the thickness of the walls surrounding the air spaces