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 from feeding on infected whitefooted mice. However, certain other species on which the larvae feed do not harbor the bacterium. Therefore, if the population of these other species were increased, the number of ticks acquiring the bacterium and hence the number of people contracting Lyme disease—would likely decline.

Which of the following, if true, most strengthens the argument?

A. Ticks do not suffer any adverse consequences from carrying the bacterium that
causes Lyme disease in humans.
B. There are no known cases of a human’s contracting Lyme disease through contact
with white-footed mice.
C. A deer tick feeds only once while in the larval stage.
D. A single host animal can be the source of bacteria for many tick larvae.
E. None of the other species on which deer tick larvae feed harbor other bacteria that
ticks transmit to humans.

Please can you explain the approach.
Thanks.

Prince,

The first step to correctly strengthening or weaking a CR argument is to properly identify the conclusion. In this case the conclusion is easily identifiable due to the indicator word "therefore." Thus, the conclusion is the entire final sentence. To strengthen this conclusion, we must support the contention that the number of people contracting Lyme disease would actually decline by increasing the population of other species on which the larvae of deer ticks feed. Use a Strengthen/Weaken Slash Chart to assess each answer choice individually.

A. Ticks do not suffer any adverse consequences from carrying the bacterium that
causes Lyme disease in humans.
-Irrelevant. The conclusion is about the number of humans that contract Lyme Disease

B. There are no known cases of a human’s contracting Lyme disease through contact
with white-footed mice.
-Irrelevant. The argument never suggests that the mice can directly infect humans.

C. A deer tick feeds only once while in the larval stage.
-Strengthen. If the larvae of a deer tick only feeds once and there are more potential uncontaminated food sources for the larvae, this supports the contention that fewer ticks would acquire the bacterium and thus infect fewer humans.

D. A single host animal can be the source of bacteria for many tick larvae.
-Weaken/Irrelevant. If anything, this answer choice suggests that multiple deer ticks can acquire the bacteria from one mouse. This definitely does not strengthen that FEWER humans would be infected.

E. None of the other species on which deer tick larvae feed harbor other bacteria that
ticks transmit to humans.
-Irrelevant. This answer choice is outside the scope of the argument. The conclusion is specifically about Lyme Disease rather than "other bacteria".

The credited response is C

-dan

Thanks Dan.
That approach really gave me some confidence.
I was split between B and C but realized my mistake after reading your post.

But IMO D actually strengthens too....as earlier they were feeding on infected food.....but they will now not eat the infected food....

For e.g. Earlier 100 deer ticks were eating say a single infected food...

But now due to more availability and less competition to food...less number of deer ticks will eat the infected food....say for example....80.....

so is it not strengthening the argument ?

Can someone please tell where is the flaw in my reasoning ?

Experts kindly share your opinion please !!!!

first of all, this is one of those "might" arguments. you're saying that fewer ticks might feed on the infected host, but there's no reason that this would have to be true.

actually, (d) could also work exactly against this possibility. i.e., since a single host can feed large numbers of deer ticks, it's reasonable that the tick population could continue to feed primarily on infected mice, and not have to spread out to the other species.

we have no way of telling which of these possibilities will happen.

There is a version of this problem with more difficult answer choices:


A. Whether populations of the other species on which deer tick larvae feed are found only in areas also inhabited by white footed mice.

B. Whether the size of the deer population is currently limited by the availability of animals for ticks ‘s larval stage to feed on

C. Whether the infected deer 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 transmits to humans.

Anyone have thoughts as to the right answer? I picked D, but my answer was incorrect. I'm having difficulty understanding why.

where did this version of the problem come from?

_________________
Tim Sanders
Manhattan GMAT Instructor

I got this problem wrong because I read the stem without jotting down notes to help me sort through the relationships and target key premise and conclusion. I decided to shift my strategy as I analyzed my mistake, by jotting down the main premise first, framed it as because (....), followed by the conclusion. Then go back to the question to understand what the heck is going on.

Not sure if it will always work but at least it helps me understand why I got a question wrong and help me find the right asnwer.

For this problem, this is what I did:

Because # of other species increases, the # of ticks decrease and the # of people get infected decrease.

I go back to the question to find out what those species are, and read the entire stem as I normally would do, then come back to the note I just took, and realized that the second part of it: "the # of ticks decrease and the # of people get infected decrease." tells me that this is not likely going to be the key issue because the causal relationship sounds pretty "well-linked", there's no loophole that stands out. So seems like what's missing is between the first half and the second half. Then it dawned on me that they're implying that/what they're really trying to claim is that "deer ticks will eat less infected white footed mice."

Now I go to choices and use negation technique on choices that follow this line of thought, and the one that seems suspicious is C, so I used negation technique on C.

A deer tick feeds only once while in the larval stage.
becomes A deer tick feeds multiple times while in the larval stage.
If that's the case then
deer ticks will NOT eat less infected white footed mice,

hence C is the answer.

I know instructors had already provided excellent explanations on this problem but I think my approach to this problem might be of help anyways. Because I tried this method on other problems I got wrong and it got easier and easier for me to find the right answer. Not sure if I can use this approach to attack a problem within 2 minutes, but maybe the more I practice it the easier and quicker it gets to apply it.

:)

_________________
Tim Sanders
Manhattan GMAT Instructor

The correct answer is actually B.

I agree with Rachel and I think diagramming as you read works best here because there are a number of relationships to account for. A sample diagram could be (its hard to draw a T-Diagram, but this is the best I could type):

Other (up arrow) -> DT eat less BACT -> Total BACT in DT (down arrow)
BACT from DT -> LymD in HU
BACT -> DT when DT Larv eat WF Mice
DT Larv also eat Other w/ no BACT

Now lets look at the answer choices:

A. Whether populations of the other species on which deer tick larvae feed are found only in areas also inhabited by white footed mice.
If Other and WF Mice are located together, it won't help you determine if the "DT eat less BACT".

B. Whether the size of the deer tick population is currently limited by the availability of animals for tick's larval stage to feed on
If the DT population is limited by the number of things they can eat, then increasing Other will enable more food for DT to eat, thereby providing a circumstance where "DT eat less BACT" as a whole.

C. Whether the infected deer tick population could be controlled by increasing the number of animals that prey on white footed mice.
This does not help us evaluate the conclusion that "DT eat less BACT".... the larvae could still be eating just as much WF 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.
This does not help us evaluate the conclusion about "Other (up arrow) -> DT eat less BACT"

E. Whether the other species on which deer tick larvae feed harbor any other bacteria that ticks transmits to humans.
Irrelevent, we are talking about Lyme Disease and how it spreads to humans

I also found this question confusing so I hope this helps.

- Alex

thanks..

_________________
Tim Sanders
Manhattan GMAT Instructor