Gas-fired and oil-fired power plants emit some pollutants into the air

B
All other options strengthen the argument except D, which inherently assumes that oil-based plants are less polluting which was never stated.

Official Explanation

This is a Weaken question. The correct choice will make the conclusion less likely to follow from the stated evidence.

The author's conclusion appears after the keyword "therefore." The author believes that in order to reduce air pollution, we should close down coal-fired plants and replace them with gas- and oil-fired plants. As evidence, the author points out that coal-fired plants emit far more noxious air pollutants than do the other two types of plants.

Note that the author is making an extreme recommendation. Closing down plants and replacing them with new plants that use different fuel is a major operation. The author must be assuming that such an extreme measure is really needed in order to reduce pollution - that there is no other solution. The correct choice will make it less likely that the coal-fired plants really should be replaced with gas- and oil-fired plants in order to reduce pollution. It might suggest some less drastic measure that would suffice or point to some unstated negatives about gas- and oil-fired plants (with regard to air pollution) that make the switch less attractive. It's hard to know exactly what the GMAT will come up with, but having the general prediction in mind will make it much easier to eliminate wrong choices.

(B) suggests an alternative, less drastic measure and is correct. If coal plants can "easily" be redesigned to eliminate most emissions, then the recommendation to replace the entire plant in order to reduce air pollution is weakened.

(A) points out another negative of coal but is irrelevant to the argument in the stimulus, which addresses only air pollution, not water pollution or all pollution.

(C) is an irrelevant comparison because the argument is about pollution, not about cost efficiency. This choice also fails to mention oil-fired plants.

(D) does not address the three fuels' effect on air pollution, so it is irrelevant to the argument in the stimulus.

(E) is a 180. It would strengthen the argument by making gas- and oil-fired plants look even more attractive in terms of emissions.­