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-

Question

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