What is the approximate percentage of oxygen in Earth's atmosphere?

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Multiple Choice

What is the approximate percentage of oxygen in Earth's atmosphere?

Explanation:
Oxygen makes up roughly one-fifth of Earth's atmosphere by volume, with the standard value about 20.95% for dry air at sea level. This means that in air, about 21% of the gas molecules are oxygen. The air is mostly nitrogen (about 78%), with the remaining fractions from argon, carbon dioxide, neon, and other trace gases. Because of these proportions, the best choice is the value closest to 21%. The other options deviate noticeably from the standard dry-air composition, either higher or lower than the typical 20.95% O2.

Oxygen makes up roughly one-fifth of Earth's atmosphere by volume, with the standard value about 20.95% for dry air at sea level. This means that in air, about 21% of the gas molecules are oxygen. The air is mostly nitrogen (about 78%), with the remaining fractions from argon, carbon dioxide, neon, and other trace gases. Because of these proportions, the best choice is the value closest to 21%. The other options deviate noticeably from the standard dry-air composition, either higher or lower than the typical 20.95% O2.

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