Which choice most logically completes the text?

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Many studies have found a positive association between levels of dissolved organic carbon and mercury in bodies of fresh water in North America. But Petri Porvari and Matti Verta did not find this correlation in a study conducted in Finland, leading some scientists to hypothesize that the association is particular to North America. However, several other studies conducted outside North America, such as one by Sara M. Ekström and colleagues in Sweden, showed similar results to the North American studies, while few have produced results similar to those of Porvari and Verta's study, suggesting that

the hypothesis that the positive association is particular to North America is correct.

levels of dissolved organic carbon and mercury in bodies of fresh water are both much higher in Finland than elsewhere.

there were circumstances unique to Ekström and colleagues' study that impeded accurate measurements of mercury levels.

dissolved organic carbon and mercury levels do typically rise and fall together in fresh water.