Passage Summary: The author argues that Dooney County isn't flat because the farms there have terraces, and terraces are only built to stop water erosion, which doesn't happen on flat land.

Conclusion: The claim that Dooney County is flat is definitely incorrect.

Reasoning: Flat land does not suffer from water erosion and thus does not require terraces, yet Dooney County farms are known to have many terraces.

Analysis: The author is relying on a specific link: the existence of terraces proves the land isn't flat. For this to work as a 'Necessary Assumption,' we must assume that the terraces were actually built to prevent water erosion rather than for some other purpose. If the farmers built them for decoration or to keep their cows from wandering off, the presence of terraces wouldn't prove anything about the flatness of the land. Look for an answer that confirms the terraces in Dooney County are specifically there to combat erosion.