27/4，结论解释型
The origin of the theory that major geologic events may occur at regular intervals can be traced back not to a study of volcanism or plate tectonics but to an investigation of marine extinctions.（理论：地理事件是周期性，这个理论起源于海洋动物的毁灭） In the early 1980’s, scientists began to look closely at the question of how these extinctions occur. Two paleontologists, Raup and Sepkoski, compiled a master list of marine species that died out during the past 268 million years and noted that there were brief periods during which many species disappeared at once. These mass extinctions occurred at surprisingly regular intervals.（RS：发现海洋动物的死亡有周期性的）
Later studies revealed that extinctions of terrestrial reptiles and mammals also occurred periodically. （后来的研究发现：陆地上的爬虫和哺乳动物灭绝也是有周期性的）These findings, combined with the research of Raup and Sepkoski, led scientists to hypothesize the existence of some kind of cyclically recurring force powerful enough to affect living things profoundly. Speculation that so powerful a force might affect geologic events as well led geologists to search for evidence of periodicity in episodes of volcanism, seafloor spreading, and plate movement.（得出假说：有个力量影响生物，也影响地质事件）
20. The passage suggests which of the following about the “force” mentioned in lines 16 and 18?
(A) It is responsible for most of the major geologic events that have occurred.
(B) It is responsible for most of the marine extinctions that have occurred.
(C) Its recurrence is unlikely to be able to be predicted by scientists.
(D) Its existence was not seriously considered by scientists before Raup and Sepkoski did their research.（D）In the early 1980’s, scientists began to look closely at the question of how these extinctions occur. Two paleontologists, Raup and Sepkoski,
(E) Its existence was confirmed by the research of Raup and Sepkoski.感觉AB都对 不知为何不是答案 谢谢作者: mary1 时间: 2004-9-13 19:26
The origin of the theory that major geologic events may occur at regular intervals can be traced back not to a study of volcanism or plate tectonics but to an investigation of marine extinctions.（理论：地理事件是周期性，这个理论起源于海洋动物的毁灭） In the early 1980’s, scientists began to look closely at the question of how these extinctions occur. Two paleontologists, Raup and Sepkoski, compiled a master list of marine species that died out during the past 268 million years and noted that there were brief periods during which many species disappeared at once. These mass extinctions occurred at surprisingly regular intervals.It is responsible for most of the marine extinctions that have occurred As a result B is not right.force powerful enough to affect living things profoundly. It is responsible for most of the major geologic events Consequently, A is not right.
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