At sea level,the blood of individuals contains approximately $5$ million red blood cells per cubic millimeter. However,individuals living at an altitude of $5400$ meters have $8$ million red blood cells per cubic millimeter. Why does this increase occur?

• A
  Due to low atmospheric $O_2$ levels,more red blood cells are required to obtain the necessary $O_2$ for survival.
• B
  There is more ultraviolet radiation,which stimulates the production of red blood cells.
• C
  People consume more nutrients,which leads to the production of more red blood cells.
• D
  People breathe pollution-free air,which allows for more oxygen intake.