It has been found that people living in very high mountains have many more RBC's in their blood than people living in plains. Which one of following reason is correct?

Options

A. The cold climate stimulates the production of RBC's to keep the bodywarm.
B. People of high mountains breathe more quickly
C. The low air pressure requires more RBC's to supply oxygen to Body's cells.
D. The low air pressure in high mountains speeds up the blood circulation so that more RBC's are needed.