Need expert help wiht this tough CR

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Need expert help wiht this tough CR

by voodoo_child » Sat Nov 12, 2011 9:58 am
Although the geological record contains some hints of major meteor impacts preceding mass extinctions, there were many extinctions that did not follow any known major meteor impacts. Likewise, there are many records of major meteor impacts that do not seem to have been followed by mass extinctions. thus the geological record suggests that there is no consistent causal link between major meteor impacts and mass extinctions.

Which one of the following assumptiosn enables the argument's conclusion to be properly inferred?

A. If there were a consistent causal link between major meteor impacts and mass extinctions, then all major meteor impacts would be followed by mass extinctions.

B. Major meteor impacts and mass extinctions cannot be consistently causally linked unless many mass extinctions have followed major meteor impacts.

C. Of the mass extinctions that did not follow any known major meteor impacts, few if any followed major meteor impacts of which the geological record contains no hints.

D. If there is no consitent causal link between major meteor impacts and mass extinctions, then not all mass extinctions could have followed major meteor impacts.

E. There could be a consistent causal link between major meteor impacts and mass extinctions even if not every major meteor impact has been followed by a mass extinction.

OA A

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Source: — Critical Reasoning |

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by tuanquang269 » Sun Nov 13, 2011 5:41 pm
I confuse between A and C :D

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by GMATGuruNY » Sun Nov 13, 2011 6:27 pm
voodoo_child wrote:Although the geological record contains some hints of
major meteor impacts preceding mass extinctions, there were many extinctions
that did not follow any known major meteor impacts. Likewise, there are many
records of major meteor impacts that do not seem to have been followed by mass
extinctions. thus the geological record suggests that there is no consistent
causal link between major meteor impacts and mass extinctions.

Which one of the following assumptiosn enables the argument's conclusion to be
properly inferred?

A. If there were a consistent causal link between major meteor impacts and mass
extinctions, then all major meteor impacts would be followed by mass
extinctions.

B. Major meteor impacts and mass extinctions cannot be consistently causally
linked unless many mass extinctions have followed major meteor impacts.

C. Of the mass extinctions that did not follow any known major meteor impacts,
few if any followed major meteor impacts of which the geological record contains
no hints.

D. If there is no consitent causal link between major meteor impacts and mass
extinctions, then not all mass extinctions could have followed major meteor
impacts.

E. There could be a consistent causal link between major meteor impacts and mass
extinctions even if not every major meteor impact has been followed by a mass
extinction.

OA A

Thanks
This passage exhibits a CHANGE IN SCOPE.
Let X = major meteor impacts and Y = mass extinctions.

Premise: In SOME cases, we have seen X without Y and Y without X.
Conclusion: There is NO consistent causal link between X and Y.
Assumption: If there are SOME cases in which X and Y are not together, then there is NO consistent causal link between X and Y.

One way to REPHRASE a conditional statement is to take its contrapositive.
To take the contrapositive, we FLIP and NEGATE: the contrapositive of If A, then B is If not B, then not A.
To flip and negate, we need to understand the following:
The opposite of NONE is SOME.
The opposite of SOME is NONE.
Thus, the contrapositive of the assumption above is as follows: If there is SOME consistent causal link between X and Y, then there will be NO cases in which X and Y are not together (implying that X and Y will ALWAYS be together).

Answer choice A gives us the contrapositive -- which is just a REPHRASING -- of the assumption: If there were a consistent causal link between major meteor impacts and mass extinctions, then all major meteor impacts would be followed by mass extinctions.

The correct answer is A.

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