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  3. 1 The solar nebula theory states that the sun, the planets, and related debris all came from a huge interstellar gas cloud calle

1 The solar nebula theory states that the sun, the planets, and related debris all came from a huge interstellar gas cloud calle

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问题 1 The solar nebula theory states that the sun, the planets, and related debris all came from a huge interstellar gas cloud called a nebula. According to the theory, the solar system formed when parts of the nebula condensed and eventually collapsed. The explosion of a nearby star may have triggered the collapse by sending shock waves into the nebula. As it collapsed, three physical processes shaped the nebula: it became hotter, it started to spin, and it flattened into a disk.
2 At first, the nebula was large and diffuse, and it rotated slowly. Gradually, the gas and dust began to condense. The densest, hottest part was at the center, where all matter existed in a gaseous state. As the gas became hotter, its atomic particles moved faster and collided more frequently, converting kinetic energy into more heat. The hot mass collected in the nebula’s center. Gradually, the forces of gravity, pressure, and magnetism caused the nebula to flatten into a spinning disk. At its core was a hot, dense protostar, surrounded by cooler regions of gas and dust. Over 50 million years, the heat and pressure at the center of the protostar became so great that hydrogen began to fuse into helium, creating an internal source of energy. At this point, the protostar "turned on" and became the sun, a main sequence star.
3 The planets formed from the gas and dust left over from the sun’s formation. The planets fall into two groups that are very different in density and composition. The inner planets, including Earth, are those in smaller orbits that are closer to the sun. They formed from heavy elements that condensed into clumps, which in turn collided with others to form larger bodies and eventually, rocky planets. The outer planets, Jupiter and those beyond, are mainly composed of gases. Hence, they are known as the gas giants: rotating spheres of gas surrounding hot, dense cores. Although the outer planets are so unlike the inner planets, they all evolved from the same cloud of gas and dust.
7. Read the first sentence of a summary of the passage. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage.
The solar nebula theory explains how the solar system formed from a huge gas cloud.

Answer Choices
(A) An exploding star near the nebula emitted energy in the form of a shock wave.
(B) The nebula condensed, grew very hot, flattened into a disk, and started to spin.
(C) In the nebula’s center was a hot, dense protostar that eventually became the sun.
(D) Main sequence stars derive their energy from the fusion of hydrogen into helium.
(E) Nebular material formed the rocky inner planets and the outer gas giants.
(F) After the planets had formed, the solar wind carried away all remaining gas.
选项
答案B,C,E
解析 B The nebula condensed, grew very hot, flattened into a disk, and started to spin: As it collapsed...it became hotter, it started to spin, and it flattened into a disk.
C In the nebula’s center was a hot, dense protostar that eventually became the sun: At its core was a hot, dense protostar...; ...the protostar "turned on " and became the sun....
E Nebular material formed the rocky inner planets and the outer gas giants: The planets formed from the gas and dust left over...; The inner planets... ... collided... to form larger bodies and eventually, rocky planets. The outer planets... are mainly composed of gases; ... are known as the gas giants....
Answers (A) and (D) are minor ideas; answer (F) is not mentioned.
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