武汉大学考博英语真题解析与专项练习-(第二版)-赠圣才学习卡20元

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《武汉大学考博英语真题解析与专项练习(第2版)》：圣才学习卡，学习一卡通，百种考试名师面授，千种科目网上辅导。提供名师网络课程 赠送圣才学习卡中华英语学习网（www.100yingyu.com）提供名师网络课程；随书赠送的圣才学习卡在圣才学习网旗下48个网站上可免费下载20元的名校考研真题和其他复习资料（历年真题、笔记讲义、在线测试等）。学习名师网络课程及下载英语类考试历年真题等相关资料可以通过以下具体途径：登录圣才学习网（www.100xuexi.com）进入中华英语学习网，或者直接登录中华英语学习网。

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Questions 5 to 8 are based on the following passage: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 are 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 is 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——atmospheres, 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 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 f