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

1. A
   people get pollution-free air to breathe and more oxygen is available
2. B
   atmospheric ${\text{O}}_2$ level is less and hence more RBCs are needed to absorb the required amount of ${\text{O}}_2$ to survive
3. C
   there is more UV radiation which enhances RBC production
4. D
   people eat more nutritive food, therefore more RBCs are formes