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What causes oxygen to diffuse into the blood from an alveolus (air sac) in the lungs?
A) The oxygen concentration in the alveolus is higher than in the atmosphere.
B) The oxygen concentration in the alveolus is lower than in the blood.
C) The oxygen concentration in the atmosphere is higher than the carbon dioxide concentration.
D) The oxygen concentration in the blood is lower than in the alveolus.