Question

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

• A. Hydrogen
• B. Carbon monoxide
• C. Nitrogen
• D. Oxygen

Answer 1

The correct option is D Oxygen

Scientists believe that the Earth was formed about 4.5 billion years ago. Its early atmosphere was probably formed from the gases given out by volcanoes. It is believed that there was intense volcanic activity for the first billion years of the Earth's existence. The early atmosphere was probably mostly carbon dioxide, with little or no oxygen. There were smaller proportions of water vapour, ammonia and methane. As the Earth cooled down, most of the water vapour condensed and formed the oceans.

So the correct option is 'oxygen'.