托福阅读真题+题目+答案：Earth’s Atmosphere

托福阅读真题+题目+答案：Earth’s Atmosphere

1 Earth’s atmosphere has changed through time. Compared to the Sun, whose composition is representative of the raw materials from which Earth and other planets in our solar system formed, Earth contains less of some volatile elements, such as nitrogen, argon, hydrogen, and helium. These elements were lost when the envelope of gases, or primary atmosphere, that surrounded early Earth was stripped away by the solar wind or by meteorite impacts, or both. Little by little, the planet generated a new, secondary atmosphere by volcanic outgassing of volatile materials from its interior.

(第1段)1. In paragraph 1, why does the author state that Earth has less nitrogen, argon, hydrogen, and helium than the Sun?

A) To argue that these elements were once part of an early atmosphere, which disappeared.

B) To suggest that these elements were drawn into the Sun’s atmosphere.

C) To provide evidence that Earth’s original atmosphere came primarily from meteorites.

D) To support the claim that Earth’s atmosphere would have changed even more if it had contained more volatile elements.

2. The word “constituent” in the passage is closest in meaning to

A) source

B) equivalent

C) component

D) product

2 Volcanic outgassing continues to be the main process by which volatile materials are released from Earth. Although it is now going on at a much slower rate. The main chemical constituent of volcanic gases (as much as 97 percent of volume) is water vapor, with varying amounts of nitrogen, carbon dioxide, and other gases. In fact, the total volume of volcanic gases released over the past 4 billion years or so is believed to account for the present composition of the atmosphere with one important exception: oxygen. Earth had virtually no oxygen in its atmosphere more than 4 billion years ago, but the atmosphere is now approximately 21 percent oxygen.

3. According to paragraph 2, the history of volcanic outgassing cannot explain which of the following?

A) The lack of oxygen in the atmosphere 4 billion years ago.

B) The amount of water vapor in the atmosphere today.

C) The proportions of nitrogen and carbon dioxide in the atmosphere today.

D) The present abundance of oxygen in the atmosphere.

3 Traces of oxygen were probably generated in the early atmosphere by the breakdown of water molecules into oxygen and hydrogen by ultraviolet light (a process called photodissociation). Although this is an important process, it cannot begin to account for the present high levels of oxygen in the atmosphere. Almost all of the free oxygen now in the atmosphere originated through photosynthesis, the process whereby plants use light energy to induce carbon dioxide to react with water, producing carbohydrates and oxygen.

4. Paragraph 3 suggests which of the following about the process of photodissociation?

A) It is more common today than it was in the early history of the atmosphere.

B) It is responsible for only a small amount of the oxygen in the atmosphere today.

C) It removes trace amounts of oxygen from the atmosphere.

D) It produces more free oxygen than photosynthesis does.

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