Birds are able to extract more oxygen than mammals from an equal volume of air. Which of the following statements reflects why birds are able to do so?

A. Birds have a greater surface area for diffusion because they have air sacs in addition to lungs.
B. Birds have air sacs that enable them to hold their breath for longer, thereby increasing the time for oxygen to diffuse.
C. Birds have capillaries that are in close contact with the lungs.
D. Birds don't have air mixing in their lungs; air flows in a unidirectional manner across the lung surface for increased diffusion.