Question

An abundance of ______ was conspicuously absent from the Earth’s atmosphere 4 billion years ago.

Options

• hydrogen

• carbon monoxide

• nitrogen

• free oxygen

Answer 1

An abundance of free oxygen was conspicuously absent from the Earth’s atmosphere 4 billion years ago.

Explanation:

The early Earth’s atmosphere was reducing, containing gases like hydrogen, nitrogen, carbon dioxide, methane, and ammonia, but it lacked free oxygen. Free oxygen started accumulating only later, after the evolution of photosynthetic aerobic organisms.