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Flaw in the Reasoning-CE. The correct answer choice is (E)
This argument is flawed because it contains a Mistaken Cause and Effect reasoning error. Specifically, the argument fails to consider that there might be an alternate explanation for an observed effect, the sudden absence of water-based life from a certain lake.
The archeological evidence described in the stimulus establishes that at a certain point in time the "water-based life was suddenly absent" from a certain lake. [Note: While the stimulus does not limit the statement that "water-based life was suddenly absent" to the lake, it is reasonable for the testtaker to assume the archaeologists did not claim water-based life was generally absent.] From this evidence, the students improperly infer that the cause of that event was the removal of water from the lake. While this causal relationship (i.e., the removal of the water caused the sudden disappearance of the water-based life) may be correct, nothing in the stimulus supports the students' inference that removal was the only potential cause of that effect.
Armed with our prephrase, that the students improperly inferred the identity of the cause from the existence of an effect, we can head to the answer choices. By skimming through the answer choices, we can see that only answer choice (E) discusses causal reasoning, and that it even uses the words "cause" and "effect." An important component of our prephrase in a Flaw in the Reasoning question is to understand the position in which the LSAT testmakers have placed themselves. By asking us to identify a reasoning error and then constructing the stimulus so that the error pertains to causality, the testmakers have created the situation in which the correct answer choice must describe in general terms how a cause interracts with an effect. There are only so many ways to do that, and therefore we can look for certain words in the answer choice, such as "cause," "effect," or any synonymous term. Answer choice (E) is correct.
consider why the remaining answer choices are incorrect. Recall that an incorrect answer choice to a Flaw in the Reasoning question will either fail the Fact Test, because the stated error did not occur in the stimulus, or it will describe something that appeared in the stimulus but did not constitute a reasoning error.
Answer choice (A): This answer choice describes an Overgeneralization from an Exceptional Case, and it fails the Fact Test. For this answer choice to be correct, we must see in the stimulus where each portion of this alleged error occurred. Since the stimulus mentions only one historical event (the recent academic activities excluded), the million person army drinking a lake dry, the stimulus does not make a generalization. An example of such a generalization could be: "The army drank the lake dry in this instance. Therefore, every time the water-based life in a lake disappears, a massive army must have emptied the lake to quench their thirst."
Answer choice (B) We can use the Fact Test to eliminate this answer choice, because the stimulus does not discuss counterevidence, let alone counterevidence that is both available and potentially useful. While language in the stimulus (e.g., the "alleged"event "really took place") expresses doubt that the army drank the lake dry, a mere expression of doubt does not amount to counterevidence.
Answer choice (C) This answer choice also fails the Fact Test, because the argument does not reject a hypothesis.
Answer choice (D) This answer choice is incorrect for two reasons. First, it is not a reasoning error to "consider people and locations whose existence cannot be substantiated by modern historians," because lack of proof does not equal proof of a lack. Here, the historians' inability to substantiate the existence of those people and locations does not prove that they did not exist. To presume they did not exist would be an Error in the Use of Evidence. Moreover, we can eliminate this answer choice by applying the Fact Test, because the archeologists apparently substantiated that the lake described in the text existed.
Answer choice (E): This is the correct answer choice. See discussion above.