What causes oxygen to diffuse from the lungs into the capillaries? A. Equal oxygen concentration in lungs and capillaries B. Oxygen...

The answer is C, the oxygen concentration is lower in the capillaries relative to the lungs.  Likewise, at the alveoli, where this transaction occurs, the concentration of carbon dioxide is higher in the capillaries and lower in the lungs, so that migrates into the lungs to be breathed out.  The oxygenated blood then gets pumped throughout the body where it gives up its oxygen to the cells.  The deoxygenated blood then returns to the lungs to become oxygenated once again.