例：It can be inferred from the passage that the first theories of grazer control mentioned in line 3 would have been more convincing if researchers had been able to,这是一个典型的改进型取非题题干，问的是第三行第一个理论怎样会更好。

　　逻辑思路：既然是改进，原文肯定说到了它的不好之处或者有unless的句子，我们把造成不好的原因取非，就是所要的答案了。

　　按照此思路定位原文三行，看到后面有这样一句话A low number of algal cells in the presence of a high number of grazers suggested, but did not prove, that the grazers had removed most of the algae.注意看插入语部分的but did not prove，这个就是缺点，取非就是prove再看五个选项：

　　 observe high phytoplankton numbers under natural lake conditions

　　 discover negative correlations between algae and zooplankton numbers from their field research

　　 understand the central importance of environmental factors in controlling the growth rates of phytoplankton

　　 make verifiable correlations of cause and effect between zooplankton and phytoplankton numbers

　　 invent laboratory techniques that would have allowed them to bypass their field research concerning grazer control

　　一眼扫过去就只有一个选项在讲证明，就是D中的verifiable。所以答案选D。很快解完此题。

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