Which of the following best describes the process of gas exchange in the lungs?
(a) Air moves in and out of the alveoli during breathing.
(b) Carbon dioxide diffuses from deoxygenated blood in capillaries into the alveolar air.
(c) Oxygen and carbon dioxide diffuse down their concentration gradients between blood and alveolar air.
(d) Oxygen diffuses from alveolar air into deoxygenated blood.
