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GMAT prep! Experts have a shot please

by bryan88 » Sun Mar 18, 2012 11:45 am
Lyme disease is caused by a bacterium transmitted to humans by deer ticks. Generally, deer ticks pick up the bacterium while in the larval stage by feeding on infected white-footed mice. However, certain other species on which the larvae feed do not harbor the bacterium. If the population of these species increased, more of the larvae would be feeding on uninfected hosts, so the number of ticks acquiring the bacterium would likely decline.

Which of the following would it be most important to ascertain in evaluating the argument?

(A) Whether populations of the other species on which deer tick larvae feed are found only in the areas also inhabited by white-footed mice.
(B) Whether the size of the deer tick population is currently limited by the availability of animals for the tick's larval stage to feed on.
(C) Whether the infected deer tick population could be controlled by increasing the number of animals that prey on white-footed mice.
(D) Whether deer ticks that were not infected as larvae can become infected as adults by feeding on deer on which infected deer ticks have fed.
(E) Whether the other species on which deer tick larvae feed harbor any other bacteria that ticks transmit to humans
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by GmatKiss » Mon Mar 26, 2012 9:27 pm
IMO: B, D is close too.
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by avik.ch » Tue Mar 27, 2012 12:14 am
refer this : https://www.manhattangmat.com/forums/cr- ... -t663.html
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by GMATGuruNY » Tue Mar 27, 2012 7:28 pm
bryan88 wrote:Lyme disease is caused by a bacterium transmitted to humans by deer ticks. Generally, deer ticks pick up the bacterium while in the larval stage by feeding on infected white-footed mice. However, certain other species on which the larvae feed do not harbor the bacterium. If the population of these species increased, more of the larvae would be feeding on uninfected hosts, so the number of ticks acquiring the bacterium would likely decline.

Which of the following would it be most important to ascertain in evaluating the argument?

(A) Whether populations of the other species on which deer tick larvae feed are found only in the areas also inhabited by white-footed mice.
(B) Whether the size of the deer tick population is currently limited by the availability of animals for the tick's larval stage to feed on.
(C) Whether the infected deer tick population could be controlled by increasing the number of animals that prey on white-footed mice.
(D) Whether deer ticks that were not infected as larvae can become infected as adults by feeding on deer on which infected deer ticks have fed.
(E) Whether the other species on which deer tick larvae feed harbor any other bacteria that ticks transmit to humans
This CR links an increase in the AMOUNT OF UNINFECTED FOOD to a decrease in the NUMBER OF INFECTED TICKS.

We are being asked to EVALUATE this link.
Thus, the correct answer must either STRENGTHEN or WEAKEN this link: it must show why INCREASING THE AMOUNT OF UNINFECTED FOOD will -- or will not -- lead to a DECREASE IN THE NUMBER OF INFECTED TICKS.

To understand the answer choices more easily, rephrase them without the word whether.

(A) The populations of the other species on which deer tick larvae feed are found only in the areas also inhabited by white-footed mice.
Doesn't help us determine whether the ticks will feed on these other species. Eliminate A.

(B) The size of the deer tick population is not currently limited by the availability of animals for the tick's larval stage to feed on.
In other words, the ticks have plenty of food; the number of ticks is not being held back by the amount of food available.
Thus, more food will NOT lead to more ticks.
Implication:
Any larvae that eat the uninfected food will not increase the total population of ticks but will instead DECREASE the number of ticks getting infected, STRENGTHENING the conclusion that the number of infected ticks will decrease.

(C) The infected deer tick population could be controlled by increasing the number of animals that prey on white-footed mice.
Outside the scope: other ways to handle the deer tick population are irrelevant. Our only concern is whether INCREASING THE POPULATION OF THE OTHER SPECIES ON WHICH THE LARVAE FEED will lead to a decrease in the number of deer ticks that get infected. Eliminate C.

(D) The deer ticks that were not infected as larvae can become infected as adults by feeding on deer on which infected deer ticks have fed.
This answer choice tries to weaken the PREMISE that deer ticks pick up the bacterium WHILE IN THE LARVAL STAGE. A premise is a FACT that cannot be disputed. Eliminate D.

(E) Whether the other species on which deer tick larvae feed harbor any other bacteria that ticks transmit to humans.
Outside the scope. The argument isn't about other bacteria.

The correct answer is B.
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by venkart89 » Tue Mar 27, 2012 8:36 pm
I am unable to understand your explanation for Option B.

I still consider Option C is correct. When the number of animals that feed on white- footed mice increases, the deer ticks have to look for an alternative. So it will definitely start feeding on other species.

Hence Option C is correct.


GMATGuruNY wrote:
bryan88 wrote:Lyme disease is caused by a bacterium transmitted to humans by deer ticks. Generally, deer ticks pick up the bacterium while in the larval stage by feeding on infected white-footed mice. However, certain other species on which the larvae feed do not harbor the bacterium. If the population of these species increased, more of the larvae would be feeding on uninfected hosts, so the number of ticks acquiring the bacterium would likely decline.

Which of the following would it be most important to ascertain in evaluating the argument?

(A) Whether populations of the other species on which deer tick larvae feed are found only in the areas also inhabited by white-footed mice.
(B) Whether the size of the deer tick population is currently limited by the availability of animals for the tick's larval stage to feed on.
(C) Whether the infected deer tick population could be controlled by increasing the number of animals that prey on white-footed mice.
(D) Whether deer ticks that were not infected as larvae can become infected as adults by feeding on deer on which infected deer ticks have fed.
(E) Whether the other species on which deer tick larvae feed harbor any other bacteria that ticks transmit to humans
If the population of these species increased, then more of the larvae would be feeding on uninfected hosts.

For this prediction to be valid, WHAT MUST BE TRUE?
That the ticks will feed on the other species if given the opportunity to do so.

To understand the answer choices more easily, rephrase them without the word whether.

(A) The populations of the other species on which deer tick larvae feed are found only in the areas also inhabited by white-footed mice.
Doesn't help us determine whether the ticks will feed on the other species. Eliminate A.

(B) The size of the deer tick population is currently limited by the availability of animals for the tick's larval stage to feed on.
Correct. This answer choice strengthens the conclusion: if the ticks don't currently have sufficient food, then they are more likely to feed on the other species.

(C) The infected deer tick population could be controlled by increasing the number of animals that prey on white-footed mice.
Outside the scope. The argument is not about whether the population can be CONTROLLED but about whether the population would INCREASE. Eliminate C.

(D) The deer ticks that were not infected as larvae can become infected as adults by feeding on deer on which infected deer ticks have fed.
This answer choice tries to weaken the PREMISE that deer ticks pick up the bacterium WHILE IN THE LARVAL STAGE. A premise is a FACT that cannot be disputed. Eliminate D.

(E) Whether the other species on which deer tick larvae feed harbor any other bacteria that ticks transmit to humans.
Outside the scope. The argument isn't about other bacteria.

The correct answer is B.
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by GMATGuruNY » Tue Mar 27, 2012 10:08 pm
venkart89 wrote:I am unable to understand your explanation for Option B.

I still consider Option C is correct. When the number of animals that feed on white- footed mice increases, the deer ticks have to look for an alternative. So it will definitely start feeding on other species.

Hence Option C is correct.
Answer choice C is irrelevant. The correct answer must help us to evaluate ONE THING AND ONE THING ONLY: whether increasing the population of the other species on which the larvae feed will prevent the ticks from becoming infected. We are not being asked to determine whether there are OTHER ways -- or BETTER ways -- to achieve this goal.

Please revisit my post above. I fleshed out the reasoning a bit.
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by HBK » Wed Mar 28, 2012 2:52 pm
Option A) seems correct. Think about it. The intention is that the deer ticks feed more on other species and less on those crazy mice. But for that it is important that the other species and mice should dwell in the same environment first.
Option C) looks tempting but think about this. If the animals feeding on those crazy mice do not belong to the other species, then those poor animals will be infected with that bacterium and may have Lyme disease. You do not want that now, do you?
Other options didn't make sense to me.
I would really like to know what others feel about this post because I am not 100% confident.
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by hey_thr67 » Fri Jun 29, 2012 9:38 am
Hi GMATGuruNY,
(D) The deer ticks that were not infected as larvae can become infected as adults by feeding on deer on which infected deer ticks have fed.
This answer choice tries to weaken the PREMISE that deer ticks pick up the bacterium WHILE IN THE LARVAL STAGE. A premise is a FACT that cannot be disputed. Eliminate D.


That's really a nice point. I want to confirm the following learning,

1- In evaluation question, Ans should affect the conclusion only.
2- Even in weaken/strengthen type of questions this kind of attack on the fact could have
wrong ?
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by GMATGuruNY » Fri Jun 29, 2012 10:06 am
hey_thr67 wrote:Hi GMATGuruNY,
(D) The deer ticks that were not infected as larvae can become infected as adults by feeding on deer on which infected deer ticks have fed.
This answer choice tries to weaken the PREMISE that deer ticks pick up the bacterium WHILE IN THE LARVAL STAGE. A premise is a FACT that cannot be disputed. Eliminate D.


That's really a nice point. I want to confirm the following learning,

1- In evaluation question, Ans should affect the conclusion only.
2- Even in weaken/strengthen type of questions this kind of attack on the fact could have
wrong ?
Correct.
A premise is a FACT that cannot be disputed.
Any answer choice that tries to weaken or strengthen a PREMISE will be incorrect.
Since the premise is a FACT that is not in dispute, it cannot be weakened, and it does not need to be strengthened.
The correct answer to a weaken/strengthen/evaluate question must weaken or strengthen the CONCLUSION.
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by ilyana » Sun Nov 17, 2013 12:54 pm
Hello!

I still don't understand it.
"the number of ticks acquiring the bacterium would likely decline" is part of the conclusion of the argument. If it were saying "proportion", then, yes, the answer choice B would be perfectly logical.

Does "the number" often mean "proportion" in problems?
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by ilyana » Sun Nov 17, 2013 1:47 pm
I've found an answer to my question in Ron's explanations (on Manhattan forum). Probably GMATGuruNY meant the same, but I didn't get it from the first read. For people who have the same question as mine above:
I was wrong: they do mean the number and not proportion.

In B if the answer is YES and the population of deer ticks is indeed limited by the availability of the animals for tick's larvae -- then NUMBER of infected ticks stays the same. New uninfected larvae go to other species, the population grows, but old mice are still hosting the same highest possible number of infected larvae. It refutes our argument.

If the answer is NO and the population is not limited by those animals "for larvae", it means that ticks don't use the full capacity of these animals. Imagine that we have five mice, five other uninfected animals and only two larvae. The probability that a larva is infected is 50%. Then we add ten more other uninfected animals. The population doesn't grow, because it's not limited by this factor. So the probability reduces to 25%, and the number of larvae infected also reduces. This is strengthening our argument.
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