Question

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

Options

• H₂

• O₂

• NH₃

• CO

Answer 1

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.