◆ American paleontologists David Raup and John Sepkoski, who have studied extinction rates in a number of fossil groups, suggest that episodes of increased extinction have recurred periodically, approximately every 26 million years since the mid-Cretaceous period. The late Cretaceous extinction of the dinosaurs and ammonoids was just one of the more drastic in a whole series of such recurrent extinction episodes. The possibility that mass extinctions may recur periodically has given rise to such hypotheses as that of a companion star with a long-period orbit deflecting other bodies from their normal orbits, making some of them fall to Earth as meteors and causing widespread devastation upon impact.
7. According to paragraph 4, what aspect of extinction episodes does the companion-star hypothesis supposedly clarify?
○Their location
○Their frequency
○Their duration
○Their severity
第四段：科学家1和2两人说大灭绝recurred periodically，自从mid-Cretaceous period开始大约26 million years就会发生一次。The late Cretaceous extinction那次也就是dinosaurs and ammonoids灭绝那次是多次大灭绝中最严重的一次。大灭绝定期发生的这种可能引出一种假设，那就是companion star 会使一些天体撞地球。
本段最主要信息就是大灭绝周期性与companion star 理论的关系，前者为后者的出现提供支持。
题干：companion star 假想能阐明大灭绝的那个方面？
根据段意，companion star 理论与大灭绝周期性也就是recur periodically有关系，整段就这两个信息，所以只能选B。至于D，恐龙灭绝说drastic，但只是个举例辅助信息，不是段落主要信息，这是不选理由之一。第二恐龙drastic，只是大灭绝中一个个例，不能推断其他灭绝的严重程度。第三，恐龙drastic与companion star 理论无直接逻辑关系，理论只能推大灭绝有周期性，不能推导出灭绝是否严重。
其实不需要考虑后两个理由，第一个理由简单明了，符合托福所见即所得，既然是事实信息题，文中一定有选项的paraphrase，找到出现数次的相应信息periodically与选项B：frequency对应即可，其它选项即可排除。如D排除困难，参考理由二即可。忽略干扰信息，抓段落主要信息，忽略修饰性信息。作者: 球球糖 时间: 2012-11-18 14:43:35