People living at sea level have around $5$ million $RBC$ per cubic millimeter of their blood,whereas those living at an altitude of $5400$ metres have around $8$ million. This is because at high altitude:

• A
  people eat more nutritive food,therefore more $RBCs$ are formed.
• B
  people get pollution-free air to breathe and more oxygen is available.
• C
  atmospheric $O_2$ level is less and hence more $RBCs$ are needed to absorb the required amount of $O_2$ to survive.
• D
  there is more $UV$ radiation which enhances $RBC$ production.