It has been found that people living in very high mountains have many more red corpuscles in their blood than people living in plains. Which one of the following best accounts for this phenomenon? (a) the cold climate stimulates the production of red corpuscles to keep the body warm (b) people of high mountains breathe more quickly (c) the low air pressure requires more red corpuscles to supply the body cells with oxygen. (d) The low air pressure in high mountains speeds up the blood circulation so that more red corpuscles are needed