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Which was absent in the atmosphere at the time of origin of life?

1. NH₃
2. H₂
3. O₂
4. CH₄

Correct answer: O₂

Solution

The earliest atmosphere had little to no free oxygen because it had not yet been generated in significant amounts by photosynthetic organisms. In contrast, gases like ammonia, hydrogen, and methane were present in the reducing atmosphere often associated with life’s origin.

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