When an old tree was felled, a very thin ring was detected among the oldest rings in the stump. When the width of rings, each corresponding to one year`s growth, was correlated with local rainfall records, it was found, as expected, that thicker rings had occurred in years with abundant rain. Therefore, the very thin ring, which dates to 1662, before rainfall records were kept, indicates that a serious drought occurred that year.
Which of the following, if true, most seriously weakens the argument given?
- Tree rings allow for a very precise and reliable way of determining the age of a tree.
- The decrease of light in rainy years was not a factor that would have limited the growth of the tree.
- Some species of trees have methods of storing water between years, and thus the thickness of their rings does not vary with year-to-year rainfall.
- There is independent evidence to suggest that the local patterns of rainfall changed significantly over the lifetime of the tree.
- Severe infestations of caterpillars can abruptly reduce the growth of trees.
Answer:
In explaining this question, we’d like to introduce a core principle that serves as the foundation for success on the Verbal section of standardized tests. This happens to be a GRE question (and a very brain-teaser-esque one), but the same principle would definitely apply to the GMAT, SAT, ACT, LSAT, etc. This core idea is what we call “The Principle of No Ambiguity.” Basically, it must be understood that Verbal questions are NOT ambiguous: there is a right answer and wrong answers and it should be black and white. Answers are not “better” or “worse.” If you find yourself in a situation in which it seems like 2 answers could both be right and that one is just “better” than the other, you are almost certainly wrong. You are probably failing to see what makes one of those answers definitively wrong. And training yourself to do this is one of the keys to success on the Verbal section (of any test).
On the above question, most people fall for choice C. At first glance, it seems to weaken the argument. But doesn’t choice E also seem to weaken the argument? It’s giving another potential reason for the lack of tree growth/the thin ring. Now, admittedly, it’s worded in a very ineffectual way (“can” abruptly reduce). But then again, so is choice C (“some species…”). This is where most people will just pick the answer that they perceive to be “better.” And sure, perhaps C feels stronger/better. But that’s a violation of The Principle of No Ambiguity! One of these answers must be right and the other wrong. This is where the REAL work begins and where the test separates the good test takers from the great.
Choice C is just wrong! The evidence in the argument tells us that for THIS tree, “when the width of rings, each corresponding to one year`s growth, was correlated with local rainfall records, it was found, as expected, that thicker rings had occurred in years with abundant rain.” So although there may be SOME trees for which “the thickness of their rings does not vary with year-to-year rainfall,” this is clearly NOT one of those trees! So choice C cannot be right! Therefore, choice E is the correct answer. Again, it posits another potential cause for the lack of growth/thinness of the ring. Could this answer be worded better, perhaps more strongly? Of course! But that would make the question easier, so this is an example of the test maker making the correct answer “deliberately suboptimal.” Choice E doesn’t weaken the argument that much. It obviously still could be a drought that caused the lack of tree growth. But by positing another potential cause, it DOES weaken the argument a little, and it is in the presence of 4 other answers that DO NOT weaken the argument in any way. This, folks, is The Principle of No Ambiguity!