Several billion years ago, which gas was the least prevalent in the atmosphere? a: nitrogenb: oxygenc: carbon dioxided: water vaporneed !

The least prevalent gas in the atmosphere was oxygen several billion years ago. This is because it was only around 2.4-3 billion years ago when the first photosynthetic bacteria evolved, meaning they produce food from inorganic compounds such as carbon dioxide and water to produce glucose and oxygen. Oxygen is then released in the atmosphere and this is called The Great Oxygenation Event. 

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