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第6部分：完形填空 (第5l～65题，每题1分，共1 5分) 阅读下面的短文，文中有15处空白，每处空白给出了4个选项，请根据短文的内容从4个选项中选择1个最佳答案，涂在答题卡相应的位置上。 Earths Inner Core Scientists have long struggled to understand what lies at the planets center, Direct observation of its center is impossible，so researchers must_____(51)to other evidence. In 1889，a German scientist detected a severe earthquake in Japan.Geophysicists concluded that shock waves_____(52)jolts(晃动)from one side of Earth through the center to the other side.Then in 1936，Danish geophysicist Inge Lehmann studied the waves______(53) to determine that within Earths core of molten(熔化了的)iron lies a solid inner core-but_____(54)that core was made of eluded(难倒)her.Other geophysicists quickly determined that Lehmanns inner core was composed mostly_____(55)iron. Since then， Lehmanns discovery has_____(56)conventional Earth science.But now scientists are challenging traditional theory with new and radical _____(57) For example，Earths center could actually contain aninner core within the inner core，claim Ishii and colleague Adam Dziewonski. Analyzing hundreds of thousands of earthquake wave _____(58),they maintain that the inner core has at its heart a tiny, even more solid sphere(球体) . This spheremay be the oldest fossil____(59)from the formation of Earth，says Dziewonski. | 
