The atmosphere that originally surrounded Earth was probably much different from the air w

The atmosphere that originally surrounded Earth was probably much different from

the air we breathe today. Earth's first atmosphere (some 4.6 billion years ago) was most

likely hydrogen and helium--the two most abundant gasses found in the universe--as

well as hydrogen compounds, such as methane and ammonia, Most scientists feel that

5 this early atmosphere escaped into space from the Earth's hot surface.

A second, more dense atmosphere, however, gradually enveloped Earth as gasses

from molten rocks within its hot interior escaped through volcanoes and steam vents.

We assume that volcanoes spewed out the same gasses then as they do today: mostly

water vapor (about 80 percent), carbon dioxide (about ten percent), and up to a few

10 percent nitrogen. These same gasses probably created Earth's second atmosphere.

As millions of years passed, the constant outpouring of gasses from the hot

interior--known as outgassing--provided a rich supply of water vapor, which formed

into clouds. Rain fell upon Earth for many thousands or years, forming the rivers,

lakes, and oceans of the world. During this Lime, large amounts of carbon dioxide were

15 dissolved in the oceans. Through chemical and biological processes, much of the carbon

dioxide became locked up in carbon sedimentary rocks, such as limestone. With much

of the water vapor already condensed into water and the concentration of carbon dioxide

dwindling, the atmosphere gradually became rich nitrogen.

It appears that oxygen, the second most abundant gas in today's atmosphere, probably

20 began an extremely slow increase in concentration as energetic rays from the sun split

water vapor into hydrogen and oxygen during a process called photodissociation. The

hydrogen, being lighter, probably rose and escaped into space, while the oxygen remained

in the atmosphere.

This slow increase in oxygen may have provided enough of this gas for primitive

25 plants to evolve, perhaps two to three billion years ago. Or the plants may have evolved

in an almost oxygen-free (anaerobic) environment. At any rate, plant growth greatly

enriched our atmosphere with oxygen. The reason for this enrichment is that plants, in

the presence of sunlight, process carbon dioxide and water to produce oxygen.

What is the main idea of the passage？

A．The original atmosphere on Earth was unstable.

B．The atmosphere on Earth has changed over time.

C．Hot underground gasses created clouds, which formed the Earth's atmosphere.

D．Plant growth depended on oxygen in the Earth's atmosphere.