In a vast ocean region, phosphorus levels have doubled in the past few

Question

In a vast ocean region, phosphorus levels have doubled in the past few decades due to agricultural runoff pouring out of a large river nearby. The phosphorus stimulates the growth of plankton near the ocean surface. Decaying plankton fall to the ocean floor, where bacteria devour them, consuming oxygen in the process. Due to the resulting oxygen depletion, few fish can survive in this region.

Which one of the following can be properly inferred from the information above?

(A) The agricultural runoff pouring out of the river contributes to the growth of plankton near the ocean surface.
(B) Before phosphorus levels doubled in the ocean region, most fish were able to survive in that region.
(C) If agricultural runoff ceased pouring out of the river, there would be no bacteria on the ocean floor devouring decaying plankton.
(D) The quantity of agricultural runoff pouring out of the river has doubled in the past few decades.
(E) The amount of oxygen in a body of water is in general inversely proportional to the level of phosphorus in that body of water.

Source: LSAT

abhinav770 · Answer

Here is my take on this question. Hope this helps!

bidskamikaze · Answer

It is safe to presume that most people will shortlist options A and E. Let us analyse these two. (A) The agricultural runoff pouring out of the river contributes to the growth of plankton near the ocean surface. In a vast ocean region, phosphorus levels have doubled in the past few decades due to agricultural runoff pouring out of a large river nearby. The phosphorus stimulates the growth of plankton near the ocean surface. Option A can be directly inferred from these two statements; CORRECT. (E) The amount of oxygen in a body of water is in general inversely proportional to the level of phosphorus in that body of water. This may seem tempting at first. But can we really infer this from the body of the passage? It is given that " phosphorus levels have doubled in the past few decades due to agricultural runoff pouring out of a large river ". But maybe this "doubling" resulted in growth of plankton. If phosphorous concentration doesn't double, then, possibly, plankton growth doesn't take place at all.. who knows?

INCORRECT A IS CORRECT.

AlexTheTrainer · Answer

Most GMAT folks know that LSAT Logical Reasoning (LR) is the progenitor of GMAT CR. But even among LSAT experts, confusion about the nature of Inference questions abound.

Some will incorrectly claim that all Inference questions ask for an answer that MUST be true based on the evidence (premises) from the stimulus.

Others will incorrectly claim that all Inference questions ask for an answer supported by the evidence from the stimulus, but not necessarily an answer that MUST be true.

And others will CORRECTLY claim that two inference question types exist: “must be true” and “supported”.

This all might seem trivial, but it is actually a very big deal. The specifics of this big deal are explained a few paragraphs below, as a defense of the claim above is required first.

The correctness of the above claim is demonstrated in a one-off 2007 publication by the writers of the LSAT: “The Official LSAT SUPERPREP”. The origins of this book actually have a bizarre backstory, beyond the scope of this reply. But it remains the only LSAT publication, published only in 2007, that “contains a guide to all three LSAT item types”.

Not surprisingly, the LSAT’s “guide”, like the GMAT’s OG, is poorly written to say the least. The following are quotes from the LSAT’s definition of the two inference question types. Note how each description is written in a different grammatical voice, making a direct comparison tricky. Of course.

.................

BEGIN QUOTE

Identifying a position that is conclusively established by information provided

Some questions test your ability to identify what follows logically from certain evidence or information. For these questions, you will be presented with information that provides conclusive support for one of the answer choices. Typical wordings for these questions include:

If the statement above are true, which of the following must also be true?

Which of the following logically follows from the statement above?

..................

Identifying a position supported by information provided

Some questions ask you to identify a position that is supported by a body of evidence, but not supported conclusively. These questions might be worded as follows:

Which of the following is most strongly supported by the information above?

Which of the following can most reasonably be concluded on the basis of the information above?

The statement above, if true most strongly support which of the following?

...................

END QUOTE

The big deal: the two Inference question types have different rules about what is allowed in the correct answer. These rules are man-made constructs designed to gain knowledge and understanding - they are NOT pure mathematical rules of the universe.

The two Inference types engage in two different types of logical reasoning: deductive reasoning and inductive reasoning. According to logicians (philosophers of logic), GMAT OG CR, and Official LSAT LR, these are the rules:

“Must be True” questions engage in deductive reasoning, so the correct answer will not introduce information not already contained in the stimulus. That is, the correct answer will NOT introduce outside information.

“Supports/supported by” questions engage in inductive reasoning, so the correct answer IS allowed to introduce information not already contained in the stimulus, although it does need to be directly related to said stimulus.

.....................

An inference question stem NOT containing the words “most”, “likely”, and “reasonable” = “must be true”.

So “can be properly inferred” = “must be true”.

So for this particular question, the correct answer will not introduce any outside information. Turns out that in this particular case, all four wrong answers introduce outside information. Can you spot this information?

Posted from my mobile device

VerbalBot · Answer

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In a vast ocean region, phosphorus levels have doubled in the past few

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In a vast ocean region, phosphorus levels have doubled in the past few

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