People living at the 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:

Options

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