The correct answer is (C).
(C) Paradox
Step 1: Identify the Question Type
The word "explain" reveals that this is a Paradox question. In this case, the paradox revolves around the comparatively greater decline of sugar maples. Identify the seemingly contradictory information, and then search for an answer that resolves the mystery.
Step 2: Untangle the Stimulus
The stimulus states that three different trees (spruce, fir, and sugar maple) all need calcium to survive and that acid rain diminishes calcium levels in the soil. However, when subjected to acid rain, sugar maples exhibit more signs of calcium deficiency than spruces or firs do.
Step 3: Make a Prediction
This makes sense if something different about sugar maples makes them more susceptible (or something about spruces and firs makes them more immune) to calcium issues caused by acid rain. It's not necessary to predict exactly what that something is, but know that the correct answer will resolve the paradox by expressing such a key difference.
Step 4: Evaluate the Answer Choices
(C) distinguishes sugar maples from the other species in a relevant way. A mineral compound found in soil is unaffected by acid rain. When calcium levels in the soil are low, spruces and firs can extract the calcium they need from that compound; sugar maples can't.
(A) brings up other ways the soil might be damaged by acid rain but doesn't address any difference between sugar maples and the other two types of trees. It's Outside the Scope of the paradox.
(B) is a 180. If sugar maples deteriorate less rapidly than spruces or firs, then it's even more mysterious why they would be the trees showing more signs of decline.
(D) makes an irrelevant distinction among different seasons instead of distinguishing the characteristics of sugar maples from those of other trees.
(E) expresses a difference between sugar maples and the other trees. However, the paradox in the stimulus concerns spruces and firs in "such forests," i.e., forests exposed to the acid rain. That spruces and firs are found in other forests as well is Outside the Scope of the paradox.
