People living at sea level have around $5 \; million$ $RBCs$ per cubic millimetre of their blood,whereas those living at an altitude of $5400 \; m$ have around $8 \; million$. This is because at high altitude:

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