Refer to the Figure Showing the Respiratory System in Birds

Multiple Choice

Refer to the figure showing the respiratory system in birds. Based on the figure, which statement about the air sacs of birds is true?

A) Most air enters the lungs having by-passed the air sacs.
B) They are primarily unpaired.
C) They provide an extra gas exchange surface.
D) Air flows from the posterior air sacs into the lungs.
E) Only the posterior air sacs are connected to the lungs.