托福考试

【单选题】Reading 4 "Four Stages of PlanetaryDevelopment"
PlanetaryDevelopment
→ In our study of the planetEarth, we will find a four-stage history of planetary development. The moon and all the terrestrial planets have passed through these stages, although differences in the way the planets were altered by these stages have produced dramatically different worlds. The moon, for example, is much likeEarth, but its evolution has been dramatically altered by its smaller size.As we explore the solar system, we will discover not entirely new processes but rather familiar effects working in slightly different ways.
The Four Stages
The first stage of planetary evolution is differentiation, the separation of material according to density.Earth now has a dense core and a lower-density crust, and that structure must have originated very early.
Differentiation would have occurred easily ifEarth were molten when it was young. Two sources of heat could have heatedEarth. First, heat of formation would be created by in-falling material.A、meteorite hittingEarth at high velocity converts most of its energy of motion into heat, and the in-falling of a large number of meteorites could release tremendous heat. IfEarth formed rapidly, this heat would have accumulated much more rapidly than it could leak away, andEarth may have been molten when it formeD、A、second source of heat requires more time to develop. The decay of radioactive elements trapped in theEarth releases heat gradually; but, as soon asEarth formed, that heat would have begun to accumulate and could have helped meltEarth to facilitate differentiation. Most ofEarth’s radioactive elements are now concentrated in the crust, where they continue to warm and soften the rock layers.
Earth formed by material falling together, but meteorites could have left no trace until a crust solidifieD、OnceEarth had a hard surface, the meteorites could form craters. This second stage in planetary evolution, cratering, was violent. The heavy bombardment was intense because the solar nebula was filled with rocky and icy debris, and the youngEarth was battered by meteorites that pulverized the newly forming crust. The largest meteorites blasted out crater basins hundreds of kilometers in diameter.As the solar nebula cleared, the amount of debris decreased, and the level of cratering fell to its present low level.Although meteorites still occasionally strikeEarth and dig craters, cratering is no longer the dominant influence onEarth’s geology.As we compare other worlds withEarth, we will discover traces of this intense period of cratering, the heavy bombardment, on every old surface in the solar system.
→ The third stage, flooding, no doubt began while cratering was still intense. The fracturing of the crust and the heating caused by radioactive decay allowed molten rock just below the crust to well up through fissures and flood the deeper basins. We will discuss such flooded basins on other worlds, such as the moon, but all traces of this early lava flooding have been destroyed by later geological activity inEarth’s crust. OnEarth, flooding continued as the atmosphere cooled and water fell as rain, filling the deepest basins to produce the first oceans.
Notice that onEarth flooding involves both lava and water, a circumstance that we will not find on most worlds.

The fourth stage, slow surface evolution, has continued for the last 3.5 billion years or more.
Earth’s surface is constantly changing as sections of crust slide over each other, push up mountains, and shift continents.
Almost all traces of the firs
网考网参考答案：D
网考网解析：
Addition is a transitional device that connects the insert sentence with the previous sentence. The reference to the way that the "mountains" and "continents" are "changing" in the previous sentence introduces the way that "air and water erode the surface and wear away geological features [like mountains and continents]" in the insert sentence. Choices A and B are not correct because they refer to flooding, not to the processes in slow surface evolution such as erosion. Choice C is not correct because the sentence does not include a reference to erosion and cannot introduce an additional sentence about erosion. document.getElementById("warp").style.display="none"; document.getElementById("content").style.display="block"; 查看试题解析出处>>