I would also pick D. The argument fails to condisder whether all bacteria came from Mars...

D.

Conclusion: Since there is only one type of bacteria on earth, Martian bacteria has died out ( implies that all bacteria on Earth are the ones that originated on Earth )

There might be a possibility that all bacteria on Earth died out and so current strain of bacteria on Earth is the one that came from Mars. D brings out this flaw.

D.

Evidence: All bacteria on earth is from "one source"
Conclusion: bacteria from Mars have died out.

Author seems to have assumed that this "one source" is "earth". Option D directly attacks this assumption, by saying that all the bacteria from earth have died. So this "one source" must be Mars.

It is theoretically possible that bacteria developed on Mars early in its history and that some were carried to Earth by a meteorite. Premise However, strains of bacteria from different planets would probably have substantial differences in protein structure that would persist over time, and no two bacteria strains on Earth are different enough to have arisen on different planets. Counter -premise
So, even if bacteria did arrive on Earth from Mars, they must have died out. - conclusion


Weaken question - find the choice that weakens the conclusion

A: It fails to establish whether bacteria actually developed on Mars - no where related to conclusion ,also author says theoretically possible

B: It fails to establish how likely it is that Martian bacteria were transported to Earth no where related to conclusion ,also author says meteroite
C: It fails to consider whether there were means other than meteorites by which Martian bacteria could have been carried to Earth -no where related to conclusion

D: It fails to consider whether all bacteria now on Earth could have arisen from transported Martian bacteria - correct ,if the bacteria on earth are arisen from mars bacteria that means mars bacteria did not die

E: It fails to consider whether there could have been strains of bacteria that originated on Earth and later died out
-no where related to conclusion

In weaken questions - look for the answer choices that attack the conclusion
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Answer is D IMO . Reason is that the argument is based on the assumption that bacteria existed on earth prior to the skirmish with meteorites carrying Martian bacteria stains. D significantly weakens the conclusion "even if bacteria did arrive on Earth from Mars, they must have died out" .

I will deeply explain only the correct choice because I'm seating at the office and do not have more time to do so.

First, the conclusion of the argument: If bacteria on Earth from Mars, they must died out.

The premise: Strains of bacteria from different planets would probably have substantial differences in protein structure that would persist over time

No two bacterial strains on Earth are different enough to have arisen on different planets.

If we insert choice E in side YES, the bacteria origin from Earth died out, then the bacteria from Mars develop. The conclusion is broken.

In side NO, reversely, the conclusion is secured.
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I go with D.

the contention made by the author is that since all bacteria on earth are not different enough to have arisen from different planets, any bacteria that may have come from mars must have died out. D refutes this by suggesting the possibility that all the bacteria we see today could be from mars and that the bacteria that originated on earth could have died out. Hence D is correct

Problems with others:

A: Unrelated
B: Unrelated
C: Unrelated
E: Even if some strains of bacteria that originated on earth did die out, this does not preclude the martian strains from dying out leaving some strains that originated on Earth. Note that the statement does not say all bacteria that originated from Earth died out, like D does.. which is what we need.

I picked E and realized why it's wrong. Who cares about strains of bacteria that originated on Earth? I picked this because I was thinking, oh well if the strains on earth died out, the bacteria that is from Mars MAY be comparable...but who cares? The conclusion is saying, so if bacteria DID come from Mars, it probably died out. D is the best choice because maybe the bacteria now IS from Mars...therefore it didn't die out (and we are assuming that the bacteria here is from Earth instead).

still I have conusion with (d) and (e)

Could anyone explain why (d) is wrong
Regards

Hi Practicegmat.

I think your question is wrong cause D is correct. Probably, your question is "why E is wrong?" if that's true, so this is my explanation.

Conclusion is: if bacteria did arrive on Earth from Mars, they must have died out because no two bacterial strains on Earth are different enough to have arisen on different planets.

What should we attack? We weaken the conclusion by showing that all bacteria on Earth now were transported to Earth from Mars and NOT died out later.

But (E) It fails to consider whether there could have been strains of bacteria that originated on Earth and later died out.


E does not attack the conclusion at all. There would be the case that some bacteria originated on Earth died out later and bacteria transported from Mars to Earth also died out.

Hence E is incorrect.

Hope it helps.

Whats wrong with D and E...

Can someone explain...?
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Hi jaituteja

I'm glad to help.

ANALYZE THE STIMULUS:

Fact: strains of bacteria from different planets  have substantial differences in protein structure that would persist over time.
Fact: NO two bacterial strains on Earth are different enough to have arisen on different planets
Conclusion: if bacteria did arrive on Earth from Mars, they must have died out.

Assumption: There is NOT ANY CASE that all bacterial strains on Earth came from Mars. Because if All bacterial on Earth originated from Mars ==> we CANNOT compare bacteria from Mars with bacteria on Earth to make conclusion, because they are the same.

D and E, which one is correct?

(D) It fails to consider whether all bacteria now on Earth could have arisen from transported Martian bacteria.
Correct. It attacks the conclusion by demonstrating the case that all bacterial strains on Earth actually traveled to Earth from Mars. we CANNOT compare bacteria from Mars with bacteria on Earth, because they are the same.

(E) It fails to consider whether there could have been strains of bacteria that originated on Earth and later died out.
Wrong. E does not weaken the point “all bacteria arrived on Earth from Mars must have died out” at all. Don’t go too FAR. You CANNOT assume that “strains of bacteria that originated on Earth and later died out” = all bacteria living on Earth arrived from Mars. Because there would be the case that some bacteria originated form Earth died out later, but others still alive. Therefore, the conclusion based on the comparison MAY BE valid ==> You cannot weaken the conclusion by using E. That's why E differs from D.

Hope it helps.

This is one of the most twister questions I have seen on GMAT. But not because of logic but sentence construction. I would have been scratching for long in understanding the meaning. Lets deconstruct.

Fact 1: It is theoretically possible that bacteria developed on Mars early in its history and that some were carried to Earth by a meteorite.

Fact 2: However, strains of bacteria from different planets would probably have substantial differences in protein structure that would persist over time, and
Not of much significance

Fact3: no two bacterial strains on Earth are different enough to have arisen on different planets.
It looked like an SC question to me for first 20 times I looked at it. But, guess GMAT is all about persistence
Let us first remove this "no" from beginning to understand the meaning: "two bacterial strains on Earth are different"
Now place "not" after "are": "two bacterial strains on Earth are not different"

So, this sentence basically says that bacterial strains on earth are so same that they must be of only one origin.

Conclusion: So, even if bacteria did arrive on Earth from Mars, they must have died out.
Bacteria from Mars, if on earth, must have died out.

Summary: We can have only one source for all bacteria on earth. So all bacteria from Mars, if reached earth, would have died out on earth.

What will weaken this? Bacteria from Mars didn't die.
D says exactly that.

E says that earth bacteria died out. It is extra information but doesnt deal with Martian bacteria(which is part of conclusion)

Correct me if this line of thinking looks wrong..

Conclusion :- Even if bacteria originated from Mars, they must have died out.

The argument makes above conclusion on the basis that strains of bacteria from different planets would have substantial differences in protein structure and no two bacterial strains on Earth are different enough to have arisen on different planets.

The argument is most vulnerable to which of the following criticisms?


Note:- In final conclusion argument agrees that bacteria may have arrived on Earth from Mars. We are not talking about whether it was developed or not on Mars.
Irrelevant


Note:- In final conclusion argument agrees that bacteria may have arrived on Earth from Mars.So this option is not the contention here.
Out of Scope.


We are not considering by what means bacteria were transported to earth.Irrelevant


This may weaken the conclusion. All bacteria that now are there on earth may be transported from Mars. They are of same protein structure. Hence the author conclusion will be weakened. Bingo


So some bacteria originated on Earth and later died out. We are still not getting as to why Mars bacteria died out even if they were successful in reaching earth. Does this option gives us the reason to weaken the conclusion above.? NO
Therefore we can cross it out as well.
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hi ,
can anyone help clarify "no two bacterial strains on Earth are different enough to have arisen on different planets. " ?

it is hard for me to understand.

thanks ini advance.

confusion between D and E.

I understand D weakens the conclusion. But to give E a thought, it is possible that all the strains of bacteria originated in earth die out, and thus the remaining ones are from Mars, doesn't that weaken the conclusion as well.

This is a pretty original thought A bit too original for the GMAT though...
While this is logically true,
It is very very very unlikely that ALL strains of bacteria on earth would die out.
And thus, this specific scenario doesn't weaken the argument very much - certainly less than D does.
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The argument is saying that all of the existing bacteria on earth is similar - too similar for any of it have come from another planet
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