A report on the likely effects of current levels of air pollution on forest growth in North America concluded that, since nitrogen is a necessary nutrient for optimal plant growth, the nitrogen deposited on forest soil as a result of air pollution probably benefits eastern forests. However, European soil scientists have found that in forests saturated with sulfate and nitrate, trees begin to die when the nitrogen deposited exceeds the amount of nitrogen absorbed by the forest system. Since this finding is likely to apply to forests everywhere, large areas of eastern forests of North America are, undoubtedly, already being affected adversely. Which one of the following most accurately expresses the main point of the passage? ['Contrary to the report cited, the nitrogen pollution now in the air is more likely to cause trees to die in eastern forests than to benefit them.', 'The implication of the report cited is that the amount of nitrogen reaching eastern forests by way of polluted air is approximately what those forests need for optimal growth.', 'The eastern forests are the only forests of North America currently affected by polluted air.', 'The type of analysis used by European soil scientists does not necessarily apply to eastern forests of North America.']