OG9 阅读 Q24

Choice A is the best answer. According to the fourth paragraph, the existence of streams in Trinidad with populations of guppies and those without guppies led Reznick to the conclusion that he could conduct experiments by altering the guppy populations in various streams. According to the second sentence of the fourth paragraph, Reznick realized he could "'treat streams like giant test tubes by introducing guppies or predators' to places they had not originally occurred, and then watch as natural selection acted on the guppies." Reznick uses the phrase "giant test tubes" in this sentence to suggest that certain streams can provide suitable experimental conditions for his guppy research.
Choices B, C, and D are incorrect because the phrase "giant test tubes" serves to characterize certain streams as suitable for experimental research, not to suggest that those streams promote cooperative behaviors among guppies used as subjects in experimental studies (choice B), increase the rate of genetic change among guppies  (choice C), or assist scientists in solving environmental problems in the natural habitat of guppies (choice D).