The Everest Base Camp in Nepal Is About 5,400 Meters

The Everest Base Camp in Nepal is about 5,400 meters above sea level, where the atmospheric pressure is about half of what it is at sea level.If not accustomed to this altitude a person can feel tired and experience shortness of breath upon exertion because

A) oxygen makes up a lower percentage of the air at this altitude than at sea level.
B) carbon dioxide makes up a higher percentage of the air than oxygen at this high altitude.
C) the partial pressure of oxygen (PO₂) at this altitude is about half of what it is at sea level.
D) the rate of diffusion of oxygen at this altitude is greater than it is at sea level.
E) there is only a small concentration gradient for diffusion of CO₂ from the body at this altitude.