Question

At
higher altitudes air becomes​

Answer 1

Explanation:

As altitude increases, the amount of gas molecules in the air decreases—the air becomes less dense than air nearer to sea level. ... Air expands as it rises, and the fewer gas molecules—including nitrogen, oxygen, and carbon dioxide—have fewer chances to bump into each other. The human body reacts to high altitudes.

Answer 2

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Rarer is the answer

At higher altitudes, the air becomes rarer and amount of oxygen per liter of air decreases.