OG9 阅读 Q23

Choice D is the best answer. In the last paragraph, the author describes an experiment in which Reznick's team removed groups of guppies from areas with large populations of pike cichlid predators and relocated them into areas above waterfalls and rapids. According to the second sentence of this paragraph, "Although small predatory killifish occurred in these new sites, these fish do not pose anything close to the danger of the cichlids." This sentence provides the best evidence for the conclusion that the streams used by Reznick's team were not entirely free of predators, as they contained populations of killifish.
Choices A, B, and C are incorrect because the cited lines don't provide the best evidence for the conclusion that the streams used by Reznick's team were not entirely free of predators. Instead, they explain the advantage for guppies living above waterfalls or rapids (choice A), outline the correlation between numbers of pike cichlids and guppy mortality rates (choice B), and explain the growing popularity of "experimental evolution" among scientists (choice C).