GRE阅读练习每日一篇(三十七)_GRE阅读

　　17. Which of the following best expresses the main idea of the passage?

　　Current theories regarding the structure of the Earth s mantle cannot account for new discoveries regarding the composition of mantle xenoliths.

　　There are conflicting hypotheses about the heterogeneity of the Earth s mantle because few mantle elements have been thoroughly studied.

　　Further research is needed to resolve the debate among geologists over the composition of the midocean ridge system.

　　There is clear-cut disagreement within the geological community over the structure of the Earth s mantle.

　　There has recently been a strong and exciting challenge to geologists long-standing belief in the heterogeneity of the Earth s mantle.

　　18. According to the passage, it is believed that oceanic islands are formed from

　　the same material as mantle xenoliths

　　the same material as the midocean ridge system

　　volcanic rocks from the upper mantle

　　incompatible elements percolating up from the lower mantle

　　mantle plumes arising from the lower mantle

　　19. It can be inferred from the passage that the supporters of the layered-mantle theory believe which of the following?

　　I. The volcanic rocks on oceanic islands are composed of material derived from the lower part of the mantle.

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