The Alveoli of the Lungs Do Not Contain Air with 20.9

Multiple Choice

The alveoli of the lungs do not contain air with 20.9% O₂ (i.e., the percentage in dry air) because

A) we normally do not ventilate our lungs at a high enough rate.
B) the lungs have too many alveoli to ventilate.
C) there is dead space in the trachea, bronchi, and lungs.
D) the trachea and bronchi are too small in volume to contain this amount of O₂.
E) some O₂ has been exchanged before reaching the alveoli.