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June 1999 LSAT
People who say that Dooney County is flat are clearly wrong. On flat land, soil erosion by water is not a problem. ...
on July 11, 2015
Could you explain this? Thanks
on July 15, 2015
Here we have a strengthen with necessary premise question. Remember that a premise is necessary for a conclusion if the falsity of the premise guarantees or brings about the falsity of the conclusion. First we check to see if the answer choice strengthens the passage, and then, if it does strengthen, we negate the answer choice to see if its negation makes the argument fall apart. If the answer choice does both those things then it is our correct answer.
Conclusion: People who say that Dooney County is flat are clearly wrong.
Why? On flat land, soil erosion by water is not a problem. As a result, farmers whose land is flat do not build terraces to prevent erosion. However, it is said that the farms in Dooney County are dotted with terraces.
Answer choice (B): "there are terraces on farmland in Dooney County which were built to prevent soil erosion."
Does this strengthen? Yes.
We are told that erosion by water is not a problem on flat land. We also know that farmers whose land is flat do not build terraces to prevent erosion.
So: if a farmer's land is flat, then they do not build terraces to prevent erosion.
FLF ==> not BTPE
BTPE ==> not FLF
Answer choice (B) is telling us that they did build terraces to prevent erosion. As you can see from our contrapositive, if we have them building terraces to prevent erosion, then the farmer's land is not flat, i.e. people who say that Dooney County is flat are clearly wrong.
Negation: there are no terraces on farmland in Dooney County, which were built to prevent soil erosion.
Does the negation make the argument fall apart? Yes.
The reasoning in the argument follows that since Dooney County is dotted with terraces and farmers whose land is flat do not build terraces to prevent erosion, then erosion is a problem in Dooney County and since on flat land, soil erosion by water is not a problem, Dooney County is clearly not flat.
However, if Dooney County has built these terraces for another reason, e.g. it likes the aesthetic look of terraces, then we cannot say that the presence of terraces leads us to believe that there is soil erosion that would not have occurred on flat land.
Thus, the negation of answer choice (B) makes the argument fall apart.
Therefore, answer choice (B) is the correct answer.
Hope that clears things up! Please let us know if you have any other questions.
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