Hopefully you’re able to spot some similarities between this question and the previous one (Fossilized Shark Teeth).

Again, we find the phrase “if true” in the question stem, so again we’re going to take the answer choices to be true. “Most help to explain” signals that we’re working with another RRE question.

For this particular question, the correct answer choice explains the phenomenon in the stimulus very thoroughly, while the four incorrect answer choices don’t explain it at all. That means the question stem didn’t need to set a lower bar of “most helps to” explain. This question stem could have read which of the following straight-up explains.

I point this out because this is the norm with easier RRE questions: the question stem sets out a low bar and the correct answer overshoots that bar. But I don’t want you to get into the habit of thinking that this pattern is the norm for all RRE questions. Harder questions will contain correct answers that are comparatively correct. They might require small, reasonable assumptions or might only partially explain the phenomena. But compared to the wrong answers, they will fare far better. Stay on alert and be prepared to do more comparisons between the less-than-idea correct answer choices and incorrect answer choices on harder RRE questions. Okay, with that in mind, let’s look at the stimulus:

When a forest is subject to acid rain, the calcium level in the soil declines (so it seems like acid rain somehow reduces calcium levels in soil). Spruce, fir, and sugar maple trees all need calcium to survive (acid rain sounds like bad news for these trees). 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.

So, the stimulus tells us that acid rain reduces calcium levels in soil. Further, that spruce, fir, and sugar maple trees all need calcium to survive. That's the setup. Then we're told that in a forest with a lot of acid rain, the sugar maples show signs of calcium deficiency more than the spruces or firs.

“Than” signals a comparison. Between what? Sugar maple trees on the one hand with spruces and firs as a group on the other. What are we comparing them on? Which one is more likely to show signs of decline consistent with calcium deficiency. So, which one wins the comparison? It’s the sugar maples! They win the shitty prize of being calcium deficient. The greater decline among sugar maples despite all three trees being susceptible to acid rain is the phenomenon the correct answer choice should explain.

Again, register your own response. For me, since I don't know a lot about trees, I had no idea why sugar maples were doing worse. I was curious. But I also wasn't naive enough to think "Wow, that's crazy. How could this possibly be? They should have shown similar reactions to acid rain!" I didn’t think that because I figured there were a host of possible explanations that I just didn’t have the knowledge to produce.

Like the previous question about sharks, I thought "Well, there's probably something different between sugar maples (shark teeth) on the one hand and spruces and firs (shark skeletons) on the other." The information tells me that all three need calcium, fine but that's where the similarity ends. How much calcium does each one need? Where do they get their calcium from? What defenses do they have against calcium deficiency? All those (and more) questions are potentially relevant.

Because there are just too many possible explanations, you shouldn’t waste your time trying to anticipate the exact right one. Rather, you should just think of a general explanation, like “sugar maples must be different from spruces and firs with respect to calcium deficiency.”

With that, let’s take a look at the answer choices:

Answer Choice (A) Soil in which calcium levels are significantly diminished by acid rain is also likely to be damaged in other ways by acid rain.

(A) Tells us that the soil where acid rain diminished calcium is likely to also be damaged in other ways. Okay, we’ll take this to be true, but it still doesn't explain why sugar maples are suffering more than spruces and firs. (A) seems to be baiting us to make further assumptions to force it to be correct, like for example that the other kinds of damage inflicted upon the soil by acid rain must somehow be responsible for the sugar maples’ greater decline, perhaps because the sugar maples are more sensitive to the other kinds of damage than spruces and firs. But it doesn’t say that, either in the stimulus or (A). That’s an assumption that is in no way supported by the stimulus provided. Think about it: How do we know that the other forms of damage acid rain inflicts upon the soil don’t disproportionately impact spruces and firs instead? We don’t. Don’t let answer choices bait you into making arbitrary assumptions!

Answer Choice (B) Sugar maples that do not receive enough calcium deteriorate less rapidly than spruces or firs that do not receive enough calcium.

(B) compares sugar maples that don’t get enough calcium to spruces and firs that don’t get enough calcium. (B) tells us that calcium-deficient sugar maples deteriorate more slowly than calcium-deficient spruces and firs. Wait, that’s a phenomenon that runs opposite of the phenomenon in the stimulus. I thought sugar maples exhibit greater signs of decline consistent with calcium deficiency. If I weren’t confused before, now I definitely am.

An edited version of (B) might say that calcium-deficient sugar maples deteriorate more quickly than calcium-deficient spruces and firs. That could offer a potential explanation: we just haven't seen the deterioration of spruces and firs yet. It'll happen, but the sugar maples deteriorate first. Maybe. This still isn’t a great explanation because it requires that we read the facts reported in the stimulus in a contrived manner. The stimulus didn't say that we saw the sugar maples decline but the spruces and firs were doing fine. The stimulus said that sugar maples were more likely to show signs of decline than the others were. This is a statement about probability, not timing. The data wasn't just prematurely reported and no amount of charitable reading will change that.

Correct Answer Choice (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.

(C) Provides the explanation. Like the correct answer in “fossilized shark teeth,” (C) makes a relevant distinction — this time not between shark teeth and skeletons, but between sugar maples v.spruces and firs. Sugar maples are different from spruces and firs in the key regard that the latter have the ability to extract calcium from mineral compounds found in the soil that are not affected by acid rain. Sugar maples cannot. That dispels the assumption that sugar maples would respond similarly to acid rain induced calcium deficiency as spruces and firs would. (C) flatly says that's false.

Answer Choice (D) Sugar maples require more calcium in the spring and summer than they do in the fall and winter.

(D) makes a comparative statement between sugar maples’ seasonal need for calcium in spring and summer v. fall and winter. Interesting, but who cares? How does this explain the difference of the effect of calcium deficiency on sugar maples v. spruces and firs described in the stimulus? (D) could have been right if the discrepancy in the stimulus was between a stand of sugar maples in spring and summer that show symptoms of calcium deficiency compared to a stand of sugar maples in the fall and winter that don’t. But the stimulus doesn’t differentiate between seasons, and it compares sugar maples to other kinds of trees. That’s very different.

Answer Choice (E) Unlike spruces or firs, most sugar maples are native to areas that receive a lot of acid rain.

(E) points out a dissimilarity, but that dissimilarity turns out to be irrelevant, i.e. it fails to explain the phenomenon. (E) says that most sugar maples are native to areas that receive a lot of acid rain whereas most spruces and firs are not. That could work as an explanation of a general phenomenon. Why are sugar maples more likely to show signs of deterioration in general? Because they're more likely to get acid rained on. That might make sense, but the stimulus isn’t talking about sugar maples v. spruces and firs generally; it’s talking about the specific group of sugar maples v. spruces and firs that cohabit the same set of forests —those that receive acid rain. The domain already specified forests that receive acid rain, so forests that don’t receive acid rain aren’t relevant.

LET'S REVIEW

Stimulus tells us that acid rain reduces calcium levels in soil. Spruce, fir, and sugar maple all need calcium to survive. But in a forest with a lot of acid rain, the sugar maples show signs of calcium deficiency more than the spruces or firs. Why? Because spruces and firs can tap into mineral reserves of calcium but sugar maples can’t.

So far, the two questions we’ve done both have correct answers that are “ideal.” But not all correct answers on RRE so strongly and completely explain the phenomena.

It’s good practice to flex your phenomenon-hypothesis muscles. Spend time coming up with specific explanations. Then see if you can generalize the explanations. Do this during review. When the clock is ticking, don’t waste time trying to predict the exact explanation. The possibilities are infinite and your knowledge is likely too limited. Spend a few seconds coming up with a general explanation. For example, you might generally surmise that sugar maples must be different from spruces and firs in some relevant respect that calcium deficiency disproportionately affects them. Then head into the answers and use the process of elimination (POE).

Correct answers will often make distinctions (disanalogize) between concepts in the stimulus. But it’s not enough for an answer choice to make a distinction; that distinction also has to be relevant to the phenomenon or discrepancy at hand.

Lessons to review
comparative