IR: Giant Kangaroos - Marco and Fatima - GMATPrep Software

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Can someone clearly explain this to me ? Though this IR (CR) question looks simple at the outset, there is fair amount of ambiguity in the answers provided. This is an official GMAT Prep Test question.

Marco: Giant kangaroos - one of several extinct species of large mammals (megafauna) - went extinct around 46,000 years ago. The deposits and wear patterns on the teeth of these animals from around the time of their extinction indicate that they fed mostly on saltbrush shrubs. Saltbrush thrives in arid climates, so it is not likely that the kangaroos' food supply was adversely affected by the increasing aridity of the climate at that time. Thus, something else would have to account for their extinction, and the best candidate for that cause is predation by humans.

Fatima: That argument alone is not likely to satisfy many researchers in this field. Have you found any other evidence to bolster your conclusion ?


Select Marco for the statement that, if true, most justifies Marco's assertions, and select Fatima, for the statement that, if true, most justifies Fatima's skepticism about Marco's assertions. Make only two selections, one in each column.


A) Giant kangaroos became extinct during a period that was less arid than previous periods they endured.

B)Many researchers believe humans first arrived in Australia around 40,000 years ago.

C)Approximately 60 different species in Australia died out in the wave of extinctions around 46,000 years ago.

D)Fossils of giant kangaroos also sho evidence that those animals' diets routinely included plants other than saltbrush.

E)Several types of megafauna larger than the giant kangaroo went extinct around 46,000 years ago.

Ans: Marco's assertions : A

Fatima's skepticism: B

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by [email protected] » Sun Dec 13, 2015 12:22 pm
s777 wrote:Can someone clearly explain this to me ? Though this IR (CR) question looks simple at the outset, there is fair amount of ambiguity in the answers provided. This is an official GMAT Prep Test question.

Marco: Giant kangaroos - one of several extinct species of large mammals (megafauna) - went extinct around 46,000 years ago. The deposits and wear patterns on the teeth of these animals from around the time of their extinction indicate that they fed mostly on saltbrush shrubs. Saltbrush thrives in arid climates, so it is not likely that the kangaroos' food supply was adversely affected by the increasing aridity of the climate at that time. Thus, something else would have to account for their extinction, and the best candidate for that cause is predation by humans.

Fatima: That argument alone is not likely to satisfy many researchers in this field. Have you found any other evidence to bolster your conclusion ?


Select Marco for the statement that, if true, most justifies Marco's assertions, and select Fatima, for the statement that, if true, most justifies Fatima's skepticism about Marco's assertions. Make only two selections, one in each column.


A) Giant kangaroos became extinct during a period that was less arid than previous periods they endured.

B)Many researchers believe humans first arrived in Australia around 40,000 years ago.

C)Approximately 60 different species in Australia died out in the wave of extinctions around 46,000 years ago.

D)Fossils of giant kangaroos also sho evidence that those animals' diets routinely included plants other than saltbrush.

E)Several types of megafauna larger than the giant kangaroo went extinct around 46,000 years ago.

Ans: Marco's assertions : A

Fatima's skepticism: B
Boil the arguments way down.

Marco: humans responsible for extinction of kangaroos, not arid climate/food shortage

Fatima: disagrees with Marco (so she doesn't think humans are responsible.)

A) This is consistent with Marco's argument. He's saying the climate wasn't responsible. If the more arid climate of past periods wasn't enough to eliminate the food supply of the kangaroo, surely a less arid climate wouldn't have caused a food shortage.

B) Fatima says the humans aren't responsible. Well, if the humans weren't there yet when the kangaroos went extinct, they're pretty clearly not the culprit.
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by happyface101 » Tue Feb 09, 2016 8:14 pm
Hi! What I don't understand is why does A support Marco's claim? The statement says that the salt shrubs thrive in arid climate, so a less arid climate could lead to the demise of the salt shrubs, thus causing a food problem and killing off the Giant Kangaroos.

Any clarity around this would be greatly appreciated. Thanks!!

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by [email protected] » Wed Feb 10, 2016 4:01 am
happyface101 wrote:Hi! What I don't understand is why does A support Marco's claim? The statement says that the salt shrubs thrive in arid climate, so a less arid climate could lead to the demise of the salt shrubs, thus causing a food problem and killing off the Giant Kangaroos.

Any clarity around this would be greatly appreciated. Thanks!!
First, there is no real indication that saltbrush shrubs would not survive in a climate less arid than some of the most arid that the kangaroos endured. The only thing said is that they thrive in arid climates.

Meanwhile, even if saltbrush shrubs were to need an arid climate in order to survive, what is said is that the climate was becoming increasingly arid at the time of the kangaroos' extinction. In other words, the kangaroos and the saltbrush shrubs were both surviving in the current level of aridity, and the climate was becoming even more arid. So if saltbrush shrubs were to indeed do better as the climate becomes more arid, things were going their way at the time of the kangaroos' extinction.
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by [email protected] » Sun Feb 14, 2016 10:15 pm
happyface101 wrote:Hi! What I don't understand is why does A support Marco's claim? The statement says that the salt shrubs thrive in arid climate, so a less arid climate could lead to the demise of the salt shrubs, thus causing a food problem and killing off the Giant Kangaroos.

Any clarity around this would be greatly appreciated. Thanks!!
You want to be careful not to assume too much (sometimes you have to, but when you can avoid doing so, avoid doing so!) We don't know HOW arid the climate needs to be for saltbrush to survive in ample enough quantities to feed these kangaroos, so we can't assume that a less arid climate means a lack of saltbrush.

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by hugoba » Mon Mar 07, 2016 6:24 am
Hi,

Isn't letter D a supporter for Marco's? If their diets routinely included plants other than saltbrush then this strenghten the conclusion that the something else, other than food supply, would have accounted for their extinction. I can't see why A is a supporter. Suppose that the opposite of A is true : "Giant Kangaroos did not became extinct during a period that was less arid than periods they endured" or "Giant Kangaroos became extinct during a period that equal in aridity or was more arid than periods they endured"; it seems to me that the opposite of A supports Marco's assertion. How can A and not(A) support the same conclusion? Is my reasoning right? Can someone please help?

Many thanks

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by [email protected] » Mon Mar 07, 2016 6:46 am
hugoba wrote:Hi,

Isn't letter D a supporter for Marco's? If their diets routinely included plants other than saltbrush then this strenghten the conclusion that the something else, other than food supply, would have accounted for their extinction. I can't see why A is a supporter. Suppose that the opposite of A is true : "Giant Kangaroos did not became extinct during a period that was less arid than periods they endured" or "Giant Kangaroos became extinct during a period that equal in aridity or was more arid than periods they endured"; it seems to me that the opposite of A supports Marco's assertion. How can A and not(A) support the same conclusion? Is my reasoning right? Can someone please help?

Many thanks
There are two problems with D. First, just because the kangaroos' diet consisted of food other than saltbrush, doesn't mean that they wouldn't have been harmed by the elimination of saltbrush. We're told that they fed "mostly on saltbrush." So this is a staple for them, even if they eat other things. Moreover, we know that the climate was becoming increasingly arid. These other foods may well have been wiped out by the increasing aridity. (saltbrush thrives in arid temperatures. But do the other plants?)

As for A, think about it this way. Marco seems to be responding to this causal argument: increase aridity ---> decrease food supply ---> kangaroos die
In other words, the arid climate is a problem for the kangaroos. Marco is saying this is wrong. In order for the original argument to make any sense, it has to be the case that this period was in fact unusually arid - otherwise, why would the food supply have been negatively impacted? But if this period is less arid than previous periods, as A stipulates, that would be consistent with Marco's claim that it wasn't the aridity that was the problem. The arid climate couldn't possibly have led to the extinction of the kangaroos if this period was less arid than previous periods.
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