Thank you for your question. This is a good example of a "main point" Logical Reasoning question. The correct answer on such questions must be true according to the stimulus and must also be the main point of the author's argument.
With this strategy in mind, let's evaluate first the stimulus and then each answer choice.
The stimulus concludes that "large areas of eastern forests of N. America are already being affected adversely" by nitrogen pollution. The premises offered in support of this conclusion are (1) although nitrogen is a necessary nutrient for optimal plant growth and (2) nitrogen deposited on forest soil as a result of air pollution can benefit forests (3) if a forest is already saturated with sulfate and nitrogen (4) trees will actually begin to die when the nitrogen deposited exceeds the amount absorbed by the forest system.
Answer choice (A) is not consistent with the stimulus, as it says something opposite (i.e., that the amount of nitrogen reaching eastern forests is what is needed for optimal forest growth; in fact, however, the stimulus argues that the amount is excessive and bound to damage those forests).
Answer choice (B) is likewise not supported by the stimulus. Nothing in the text of the argument supports this proposition, that is, that forest capacity for absorbing nitrogen will increase if the forest is oversaturated with nitrogen.
Answer choice (C), again, is contrary to the stimulus. The stimulus says "since this finding is likely to apply to forests everywhere." Yet answer choice (C) implies that the analysis "does not necessarily apply."
There is no textual support for answer choice (D). Put differently, there is no indication that this answer choice "must be true" according to the text of the stimulus.
Answer choice (E) is both textually supported AND an expression of the author's main point. "Contrary to the report cited"--that is, the report mentioned at the beginning of the stimulus, which said that nitrogen can benefit forests--nitrogen air pollution is more likely to kill the eastern forest trees than benefit them--in other words, such pollution is "adversely affecting" the eastern forests.
Hope this helps! Please let us know if you have any other questions.
