Why is the concentration of oxygen in the body cells lower than the concentration of oxygen in the alveolus? A. Alveolus can produce oxygen.
B. The exchange of gases occur in the alveolus.
C. Oxygen is used up for respiration I the body cells.
S. Oxygen diffuses out from the body cells.

i think the answer should be b.

the alveolus is the site of exchange of gases (carbon dioxide and oxygen) between the body cells and the alveolar air. diffusion requires a concentration gradient, which basically means that there has to be a difference in concentration of the substance (in this case it's oxygen) between the 2 areas (in this case it's the body cells and the alveolar air) hence, there is more oxygen in the alveolar air than the body cells (this difference in concentration is what allows diffusion of oxygen from a region of HIGHER to LOWER concentration!) this allows oxygen to diffuse from alveolus to body cells, giving your body the oxygen it needs!

btw keep in mind that i am by no means an expert in biology (because im still a student), but b is the most logical answer to me. good luck!