Why does the RBC count rise in people who live at high altitudes?

Due to the high partial pressure of oxygen at high altitudes, more RBCs are required to supply oxygen to the body cells.

Due to the low partial pressure of oxygen at high altitudes, more RBCs are required to supply oxygen to the body cells.

Due to the low partial pressure of oxygen at high altitudes, more RBCs are required as the speed of blood circulation increases.

Due to the high partial pressure of oxygen at high altitudes, more RBCs are required as the speed of blood circulation increases.