During Respiratory Exchange, Carbon Dioxide Moves from the Blood into the Alveoli

During respiratory exchange, carbon dioxide moves from the blood into the alveoli due to:

A) the presence of a permease, which removes carbon dioxide from the capillaries.
B) potassium cotransport.
C) diffusion down a concentration gradient.
D) the air pressure of dissolved oxygen, which forces it out.
E) the contraction of the diaphragm, which creates a vacuum that pulls it out.