The geology of the Earth’s surface is dominated by the particular properties of water. Present on Earth in solid, liquid, and ga

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问题 The geology of the Earth’s surface is dominated by the particular properties of water. Present on Earth in solid, liquid, and gaseous states, water is exceptionally reactive. It dissolves, transports, and precipitates many chemical compounds and is constantly modifying the face of the Earth.
Evaporated from the oceans, water vapor forms clouds, some of which are transported by wind over the continents. Condensation from the clouds provides the essential agent of continental erosion: rain. Precipitated onto the ground, the water trickles down to form brooks, streams, and rivers, constituting what are called the hydrographic network. This immense polarized network channels the water toward a single receptacle: an ocean. Gravity dominates this entire step in the cycle because water tends to minimize its potential energy by running from high altitudes toward the reference point, that is, sea level.
The rate at which a molecule of water passes through the cycle is not random but is a measure of the relative size of the various reservoirs. If we define residence time as the average time for a water molecule to pass through one of the three reservoirs (atmosphere, continent, and ocean), we see that the times are very different. A water molecule stays, on average, eleven days in the atmosphere, one hundred years on a continent and forty thousand years in the ocean. This last figure shows the importance of the ocean as the principal reservoir of the hydrosphere but also the rapidity of water transport on the continents.
A vast chemical separation process takes place during the flow of water over the continents. Soluble ions such as calcium, sodium, potassium, and some magnesium are dissolved and transported. Insoluble ions such as aluminum, iron, and silicon stay where they are and form the thin, fertile skin of soil on which vegetation can grow. Sometimes soils are destroyed and transported mechanically during flooding. The erosion of the continents thus results from two closely linked and interdependent processes, chemical erosion and mechanical erosion. Their respective interactions and efficiency depend on different factors.
The first paragraph mainly________.

选项 A、talks about the various forms water exists on the Earth
B、details how our geology is dominated by chemical compounds and water
C、discusses the importance of water to Earth
D、explains the relationship between water and different chemical compounds

答案C

解析主旨题。第一段提到了水对地球表面的影响，即水不断地改变地球表面。A项“讨论地球上水的不同存在形式”和D项“解释水和不同化合物之间的关系”过于片面；B项“详细解释我们的地质是如何被化合物和水影响的”，第一段并没有详细介绍，details表述错误。故本题选C项“讨论水对于地球的重要性”。

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