In the Above Graph,what Does the a Represent

Multiple Choice

In the above graph,what does the A represent?

A) anatomical dead space
B) air that helps keep the alveoli inflated (open) after forced exhalation
C) air that is not used for gas exchange
D) air with a concentration of oxygen higher than that of air outside the body
E) air from a previous breath that does not mix with newly-inhaled air of the next breath