Which of the following is not true of the respiratory tract from the medium bronchi to the aveoli?
a. Lining of the tubes changes from ciliated columnar to simple squamous epithelium in the alveoli.
b. Proportionally, smooth muscle decreases uniformly.
c. Resistance to air flow increases due to the increase in cross-sectional diameter
d. Cartilage gradually decreases and disappears at the bronchioles.