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. there is more UV radiation which enhances RBC production
• D. atmospheric O2 level is less and hence more RBCs are needed to absorb the required amount of O2 to survive