Which of the following gases probably least abundant in the early atmosphere?

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KollegeApply · Accepted Answer

O2 Explanation: The early Earths atmosphere was reducing, lacking free oxygen. Gases such as hydrogen (H2), ammonia (NH3), and carbon monoxide (CO) were present in significant amounts, but oxygen was conspicuously absent because it only began accumulating after the evolution of photosynthetic aerobic organisms.

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