(c) : At high altitudes composition of air remains almost same as at sea level, but density (barometric pressure) of air gradually decreases due to which arterial $pO_2$ is also decreased (hypoxemia). Highaltitudes presents with complex conditions to whichhuman body has to acclimatize. Number of red blood cells per unit volume of blood is likely to be higher in a person living at high altitudes. This is in response to the air being less dense at high altitude. More number of red blood cells are needed to trap $O_2$ from rarefiedair having low $pO_2$ (partial pressure of oxygen).