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  3. The atmosphere that originally surrounded Earth was probably much different from the air we breathe today. Earth’s first at

The atmosphere that originally surrounded Earth was probably much different from the air we breathe today. Earth’s first at

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问题          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.
选项 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.
答案B
解析
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