1. carbon dioxide
  2. nitrogen
  3. oxygen
  4. hydrogen
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Answer: Option 2

Below a height of about 86 kilometers, the three main gaseous elements, which together account for about 99.9% of the total atmosphere, exist in essentially constant proportion to the total: nitrogen comprises 78% of air by volume, oxygen comprises 21% of air by volume, and argon comprises another 0.9%.

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