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

Options

A. atmospheric `O_2` level is less and hence more RBCs are needed to absorb the required amount of `O_2` to survive
B. there is more UV radiation which enhances RBC production
C. people eat more nutritive food, therefore more RBCs are formed
D. people get pollution - free air to breathe and more oxygen is available

Answer

A