The primary reason it is harder to breathe when climbing up a high mountain is the air is thinner. At higher altitudes, the atmospheric pressure decreases, which means there are fewer air molecules (including oxygen) in a given volume of air. As you ascend, the oxygen availability diminishes, making it more challenging for your body to obtain the oxygen it needs.
The other options you mentioned do not accurately describe the conditions at high altitudes:
- The air compresses the lungs: This is not correct; rather, the lungs expand more to accommodate the thinner air.
- The air has more moisture: Generally, air tends to be drier at higher altitudes.
- The air pressure increases: This is incorrect; air pressure decreases as you ascend to higher altitudes.
Therefore, the correct answer is that the air is thinner, leading to a lower concentration of oxygen, which is why breathing becomes more difficult at high elevations.