3. 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.（D）
(E) Aside from harmful emissions from the smokestacks of its plants, the activities of Northern Power do not cause significant air pollution.
为什么是D呢？这个为什么是假设？我选的是E！求指点！ | 
