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PASSAGE 12

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the

correct answer to each of the questions from 1 to 8

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 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 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 out gassing

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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 time, large amounts of

carbon dioxide were 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 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 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.

Question 1. 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.

Question 2. The word "enveloped" in the second paragraph is closest in meaning to

A. escaped

B. changed

C. surrounded

D. characterized

Question 3. According to the passage, out gassing eventually led to all of the following EXCEPT

A. increases in the carbon dioxide content of sedimentary rocks

B. the formation of clouds

C. the formation of bodies of water

D. decreases in the level of nitrogen