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 avialable
B. Atmospheric oxygen level is less and hence more RBCs are needed to absorb required amount of `O_2`
C. There is more UV-radiation which enhance RBC production
D. People eat more nutritive food,therefore more RBCs are formed