Hovkial
When a forest is subject to acid rain, the calcium level in the soil declines. Spruce, fir, and sugar maple trees all need calcium to survive. However, sugar maples in forests that receive significant acid rain are much more likely to show signs of decline consistent with calcium deficiency than are spruces or firs in such forests.
Which one of the following, if true, most helps to explain the greater decline among sugar maples?
(A) Soil in which calcium levels are significantly diminished by acid rain is also likely to be damaged in other ways by acid rain.
(B) Sugar maples that do not receive enough calcium deteriorate less rapidly than spruces or firs that do not receive enough calcium.
(C) Spruces and firs, unlike sugar maples, can extract calcium from a mineral compound that is common in soil and is not affected by acid rain.
(D) Sugar maples require more calcium in the spring and summer than they do in the fall and winter.
(E) Unlike spruces or firs, most sugar maples are native to areas that receive a lot of acid rain.
Correct answer: C
A: Incorrect. (A) does not differentiate between the effect acid rain has on sugar maples from the effect that it has on spruces and firs. That acid rain may have additional effects beyond calcium depletion does not explain the faster deterioration of sugar maples.
B: Incorrect. (B) does not explain the phenomenon but rather obfuscates it. If sugar maples that do not receive enough calcium deteriorate less rapidly than do similarly deficient spruces and firs, then the opposite conclusion would be reached.
C: Correct. (C) explains how spruces and firs might be able to withstand calcium depletion caused by acid rain better than can sugar maples, since they have access to supplies of calcium that are withheld from sugar maples.
D: Incorrect. There is no mention of when acid rain principally occurs or of when spruces and firs require most calcium. The addition of information about sugar maples is irrelevant to the determination that they deteriorate faster in calcium-deficient environments.
E: Incorrect / out of scope. Whether sugar maples are native to the regions that receive a lot of acid rain does not explain why they deteriorate more rapidly.