========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. 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.
Later studies revealed that extinctions of terrestrial reptiles and mammals also occurred periodically. 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.
17. According to the passage, Raup and Sepkoski’s research was concerned with
(A) learning more about the habitats of marine species
(B) studying plate tectonics and the occurrence of volcanism over the past 268 million years
(C) examining extinctions of marine species over the past 268 million years
(D) finding out whether a rhythmically recurring geologic force exists
(E) confirming previous evidence suggesting that extinction of terrestrial species occurred regularly
18. The author of the passage would most likely describe the findings of Raup and Sepkoski as
(A) plausible, because the findings supported the theories of previous researchers
(B) significant, because the findings were an impetus for subsequent research
(C) controversial, because the findings contradicted the theories of previous researchers
(D) questionable, because the authors were not working in their field of expertise
(E) definitive, because the findings confirmed the existence of a rhythmically recurring force
19. The author of the passage is primarily concerned with
(A) determining the dates of various geologic events
(B) defending the conclusions reached by Raup and Sepkoski
(C) establishing a link between the disciplines of paleontology and geology
(D) proving that mass extinctions of marine animals occur periodically
(E) explaining how a theory concerning geologic events was formulated
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.
(E) Its existence was confirmed by the research of Raup and Sepkoski.