PASSAGE 2

　　The geology of the Earth's surface is dominated by the particular properties of water. Presenton 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 ofthe Earth.

　　Evaporated from the oceans, water vapor forms clouds, some of which are transported bywind over the continents. Condensation from the clouds provides the essential agent ofcontinental erosion: rain. Precipitated onto the ground, the water trickles down to form brooks,streams, and rivers, constituting what are called the hydrographic network. This immensepolarized network channels the water toward a single receptacle: an ocean. Gravity dominatesthis entire step in the cycle because water tends to minimize its potential energy by running fromhigh altitudes toward the reference point, that is, sea level.

　　The rate at which a molecule of water passes though the cycle is not random but is a measureof the relative size of the various reservoirs. If we define residence time as the average time for awater 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 inthe atmosphere, one hundred years on a continent and forty thousand years in the ocean. Thislast figure shows the importance of the ocean as the principal reservoir of the hydrosphere butalso the rapidity of water transport on the continents.

　　A vast chemical separation process takes places during the flow of water over the continents.Soluble ions such as calcium, sodium, potassium, and some magnesium are dissolved andtransported. Insoluble ions such as aluminum, iron, and silicon stay where they are and form thethin, fertile skin of soil on which vegetation can grow. Sometimes soils are destroyed andtransported mechanically during flooding. The erosion of the continents thus results from twoclosely linked and interdependent processes, chemical erosion and mechanical erosion. Theirrespective interactions and efficiency depend on different factors.

　　1. The word modifying in line 4 is closest in meaning to

　　(A) changing

　　(B) traveling

　　(C) describing

　　(D) destroying

　　2. The word which in line 5 refers to

　　(A) clouds

　　(B) oceans

　　(C) continents

　　(D) compounds

　　3. According to the passage , clouds are primarily formed by water

　　(A) precipitating onto the ground

　　(B) changing from a solid to a liquid state

　　(C) evaporating from the oceans

　　(D) being carried by wind

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