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-

1. A. People get pollution-free air to breathe and more oxygen is available
2. B. Atmospheric O2 level is less and hence more RBCs are needed to absorb the required amount of O2 to survive
3. C. There is more UV radiation which enhances RBC production
4. D. People eat more nutritive food, therefore more RBCs are formed

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