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掌握了gre阅读里的长难句，到了实战演习的时候了。gre阅读练习每日一篇帮助gre考生循序渐进地进行练习和总结。希望gre考生在进行gre阅读练习时，也按着考试时候的时间规定自己的练习，这样才能有效果。 Theorists are divided concerning the origin of the Moon. Some hypothesize that the Moon was formed in the same way as were the planets in the inner solar system from planet-forming materials in the presolar nebula. But, unlike the cores of the inner planets, the Moons core contains little or no iron, while the typical planet-forming materials were quite rich in iron. Other theorists propose that the Moon was ripped out of the Earths rocky mantle by the Earths collision with another large celestial body after much of the Earths iron fell to its core. One problem with the collision hypothesis is the question of how a satellite formed in this way could have settled into the nearly circular orbit that the Moon has today. Fortunately, the collision hypothesis is testable. If it is true, the mantlerocks of the Moon and the Earth should be the same geochemically. 17. The primary purpose of the passage is to present two hypotheses concerning the origin of the Moon discuss the strengths and weaknesses of the collision hypothesis concerning the origin of the Moon propose that hypotheses concerning the Moons origin be tested | 
