There is decreased pressure on the air molecules as one moves higher and the air is thinner
We know that the air pressure decreases with height. This implies that the air is thinner as you move up to higher altitude. This is the idea behind the use of the phrase; "Thin air".
Now, owing to the fact that there is decreased pressure on the air molecules as one moves higher and the air is thinner, it is much harder to breath until her aunt adjusts to the higher altitude.
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