Correct Option: A

Low air pressure is usually the most significant limiting factor in high mountain regions. The percentage of oxygen in the air at 3.2 km is essentially the same as at sea level (21%). However, the air pressure is 30% lower at the higher altitude due to the fact that the atmosphere is less dense—that is, the air molecules are farther apart. At high altitudes, the lower air pressure makes it more difficult for oxygen to enter our vascular systems. The result is hypoxia, or oxygen deprivation. In serious cases, pneumonia-like symptoms (pulmonary edema) due to hemorrhaging in the lungs and an abnormal accumulation of fluid around the brain (cerebral edema) develop. Pulmonary and cerebral edema usually results in death within a few days if there is not a return to normal air pressure levels. There is also an increased risk of heart failure due to the added stress placed on the lungs, heart, and arteries at high altitudes.