Explaining a Critical Reasoning Discrepancy

Stacey Koprince —  July 2, 2013 — 2 Comments

beetleGoing for 90th+ percentile on verbal? Chances are you’ve spent most of your Critical Reasoning study time focused on the major question types, and that is a good place to start. If you’re going for a very high verbal score, though, then you’ll also need to tackle the minor question types.

That’s what we’re going to do today; below is a GMATPrep CR problem. Give yourself about 2 minutes to do this problem.

In Asia, where palm trees are non-native, the trees’ flowers have traditionally been pollinated by hand, which has kept palm fruit productivity unnaturally low. When weevils known to be efficient pollinators of palm flowers were introduced into Asia in 1980, palm fruit productivity increased”by up to 50 percent in some areas”but then decreased sharply in 1984.

Which of the following statements, if true, would best explain the 1984 decrease in productivity?

(A) Prices for palm fruit fell between 1980 and 1984 following the rise in production and a concurrent fall in demand.

(B) Imported trees are often more productive than native trees because the imported ones have left behind their pests and diseases in their native lands.

(C) Rapid increases in productivity tend to deplete trees of nutrients needed for the development of the fruit-producing female flowers.

(D) The weevil population in Asia remained at approximately the same level between 1980 and 1984.

(E) Prior to 1980 another species of insect pollinated the Asian palm trees, but not as efficiently as the species of weevil that was introduced in 1980.

(Note: if you aren’t yet familiar with the 4-step process for answering CR questions, take a look at this article.)

Step 1: Identify the Question

This question stem contains the classic clues for an Explain a Discrepancy question. These questions, like Strengthen and Weaken questions, typically include the words if true (or the equivalent). Further, the question literally asks what would explain something.

Step 2: Deconstruct the Argument

All right, this is a Discrepancy question. The argument won’t contain a conclusion. It’ll contain some facts, at least one of which is surprising in some way. It won’t fit with the rest of the information.

Here’s what I thought and wrote while I did the problem. Your own thought process won’t be exactly the same as mine and, of course, your notes will probably look quite different, since we all have our own ways of abbreviating things. (Note: ED = Explain Discrepancy)

The Argument

What I Write

What I’m Thinking

In Asia, where palm trees are non-native, the trees’ flowers have traditionally been pollinated by hand, which has kept palm fruit productivity unnaturally low. ED     A  B  C  D  EP hand-pollin → â†“ prod Interesting”pollinating by hand must be tough. I’m going to guess that both Asia and non-native are pieces of background info, but I’ll keep them in mind as I continue to read, in case I need to write them down.
When weevils known to be efficient pollinators of palm flowers ED     A  B  C  D  EP hand-pollin → â†“ prod Some animals? insects? let’s call them animals* some animals can do a better job of pollinating these palm flowers. That’s good.
were introduced into Asia in 1980, palm fruit productivity increased”by up to 50 percent in some areas”but then decreased sharply in 1984. ED     A  B  C  D  EP hand-pollin → â†“ prodW → >> prod 1980BUT ↓↓ 1984 And they had the expected result at first. But then why did productivity decrease sharply four years later? There must be a reason.
Maybe a lot of the weevils died off for some reason?

*Weevils are beetles. I pretended I didn’t know because you don’t really need to know that; don’t worry if you don’t know what weevils are and don’t waste time deciding what to call them in your mind!

Step 3: State the Goal

This is a Discrepancy question, so I have to find the thing that doesn’t fit in the argument and figure out what new piece of information would help the argument make sense.

In this case, the introduction of the weevils worked at first, but then production dropped again for no apparent reason. There must be some reason why, and I’ve already brainstormed one: maybe something happened in 1984 to cause a bunch of the weevils to die, so they were no longer able to pollinate the trees at the same level.

Work from Wrong to Right

 

Answer Choice

What I Write

What I’m Thinking

(A) Prices for palm fruit fell between 1980 and 1984 following the rise in production and a concurrent fall in demand. ED     A  B  C  D  EP hand-pollin → â†“ prodW → >> prod 1980BUT ↓↓ 1984 This answer choice goes along with the other details of the story; it would make sense that prices fell when production rose. But this doesn’t explain why production dropped in 84. Not it.
(B) Imported trees are often more productive than native trees because the imported ones have left behind their pests and diseases in their native lands. ED     A  B  C  D  EP hand-pollin → â†“ prodW → >> prod 1980BUT ↓↓ 1984 Okay I have no idea whether the trees were imported or native. The weevils were imported but they aren’t trees. This one would only work if they told me that they replaced all of the imported trees with native trees or something. Not it.
(C) Rapid increases in productivity tend to deplete trees of nutrients needed for the development of the fruit-producing female flowers. ED     A  B  C  D  EP hand-pollin → â†“ prodW → >> prod 1980BUT ↓↓ 1984 That’s interesting. So, for four years, fruit productivity increased by up to 50 percent. I think that can reasonably be described as rapid. If that then depletes the trees of necessary nutrient to continue being productive, then this explains why the trees did well for a few years and then production suddenly dropped.
(D) The weevil population in Asia remained at approximately the same level between 1980 and 1984. ED     A  B  C  D  EP hand-pollin â†’ â†“ prodW â†’ >> prod 1980BUT â†“↓ 1984 This is the opposite of what I was thinking before: that the weevil population dropped in 84. If it stayed the same, then the story is even more of a mystery: why did production suddenly drop? Eliminate D.
(E) Prior to 1980 another species of insect pollinated the Asian palm trees, but not as efficiently as the species of weevil that was introduced in 1980. ED     A  B  C  D  EP hand-pollin â†’ â†“ prodW â†’ >> prod 1980BUT â†“↓ 1984 I’m trying to figure out why production dropped in 84, not what happened before 1980. Not it.

 

The correct answer is C.

Notice that wrong answer (D) did address the weevil population”just not in a way that would help explain the story. Answer (A) is also tricky. It provided a detail that made sense with the story, but that detail didn’t answer the specific question asked: why productivity decreased in 84.

 

Take-aways for CR Discrepancy questions:

(1) The question stem will usually contain the words if true along with a request to explain something.

(2) Once you realize you have a Discrepancy question, look for the surprising event or disconnect in the argument. The correct answer is going to hinge on this!

(3) Watch out for trap answers that do go along with the argument but don’t specifically explain the discrepancy (like answer A in this problem). Also watch out for trap answers that seem to address the detail at issue, but instead just make the surprising event even more surprising (like answer D in this problem).

 

* GMATPrep questions courtesy of the Graduate Management Admissions Council. Usage of this question does not imply endorsement by GMAC.

Stacey Koprince

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Stacey Koprince is an Instructor and Trainer as well as the Director of Online Community for Manhattan Prep. She also co-manages the company's GMAT curriculum and product line. She has been teaching various standardized tests for more than fifteen years and her entire teaching philosophy can be summed up in five words: teaching students how to think.

2 responses to Explaining a Critical Reasoning Discrepancy

  1. Hi Stacy,

    Imported trees are often more productive than native trees because the imported ones have left behind their pests and diseases in their native lands.

    Coming back to the explanation for choice B. It’s stated in the question stem that Palm trees are non-natives to Asia and hence were pollinated manually. This implies that Palm Trees were imported from somewhere other than Asia. Hence in this context, choice B intends to test us weather there were other external factors which have affected the increase in productivity and subsequent decline like an external pest.

    Choice B in this case seems fairly close. However, we can eliminate choice B since it does not account for increase and decrease in productivity of palm trees. If at all there was a foreign infestation it should have affected the productivity from the start.

    Vaibhav.

    • Another point of eliminating choice B would be to understand that increase and decrease of productivity of palm trees, was only when beetles were introduced. If choice B would have been valid, there would have been fair bit of productivity prior to the introduction of beetles since native pests of palm trees were left behind and are not present in Asia.

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