Humans Cannot Survive Long at Altitudes Greater Than 6000 M

Humans cannot survive long at altitudes greater than 6000 m because ________.

A) alveoli become compressed at the lower atmospheric pressures
B) there is much more CO₂ in the atmosphere at the higher altitudes than at sea level
C) the partial pressure of O₂ becomes less than half that at sea level
D) the diaphragm cannot be contracted enough to produce a full breath