19. The hot spot theory may prove useful in explaining _____ .

　　[A] the structure of the African plates

　　[B] the revival of dead volcanoes

　　[C] the mobility of the continents

　　[D] the formation of new oceans

　　20. The passage is mainly about _____ .

　　[A] the features of volcanic activities

　　[B] the importance of the theory about drifting plates

　　[C] the significance of hot spots in geophysical studies

　　[D] the process of the formation of volcanoes

　　[第17题答案及题解页数] B 第页

　　[第18题答案及题解页数] B 第页

　　[第19题答案及题解页数] D 第页

　　[第20题答案及题解页数] C 第页

　　[长难句突破]

　　Unlike most of the world’s volcanoes, they are not always found at the boundaries of the great drifting plates that make up the earth’s surface; on the contrary, many of them lie deep in the interior of a plate.

　　主体句式：They are not always found…; many of them lie deep…

　　结构分析：本句由分号组成了两个部分。第一个分句中，unlike引导的壮语起到了比较的作用，主句中包含了一个that引导的定语从句、修饰plates。第二个分句中，on the contrary表示了与上文的转折关系。

　　As the dome grows, it develops seed fissures (cracks); in at least a few cases the continent may break entirely along some of these fissures, so that the hot spot initiates the formation of a new ocean.