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Installing scrubbers in smokestacks and switching to cleaner-burning fuel are the two methods available to Northern Power for reducing harmful emissions from its plants. Scrubbers will reduce harmful emissions more than cleaner-burning fuels will. Therefore, by installing scrubbers, Northern Power will be doing the most that can be done to reduce harmful emissions from its plants.

Which of the following is an assumption on which the argument depends?

(A)          Switching to cleaner-burning fuel will not be more expensive than installing scrubbers.

(B)         Northern Power can choose from among various kinds of scrubbers, some of which are more effective than others.

(C)         Northern Power is not necessarily committed to reducing harmful emissions from its plants.

(D)         Harmful emissions from Northern Power's plants cannot be reduced more by using both methods together than by the installation of scrubbers alone.

(E)         Aside from harmful emissions from the smokestacks of its plants, the activities of Northern Power do not cause significant air pollution.

答案是D, 题目中没有提到只装scrubbers啊？怎么选项比较的是两者都有与只装scrubbers

与单独装scrubbers相比，两个都装上不会减少更多的有害气体。

因此，只用scrubber就足够了。

作假设题，有一种方法，就是把选项去掉not,看是否是对原结论的削弱，如果是，那么就是正确的假设，假设是必要条件，而非充分条件。此题去掉not，就是两个方法和起来更有效，是对原结论的削弱

看原文的提干，主要的议题就是怎样减少有害气体的排放，围绕这个就是正确选项。

楼上的方法不错啊！

D选项其实很合理。文章说：要完成一件事有X和Y两种方法，X方法比Y方法效率更高，因此，只采用X方法就实现了最大的效率。问假设。我们不禁要疑问，要是两种方法同时采用呢？因此D选项填补了这个漏洞。