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 Andrea G. Bravo and colleagues did not find this correlation in a study conducted in Sweden, 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 Clara E. Moreno and colleagues in Norway, showed similar results to the North American studies, while few have produced results similar to those of Bravo and colleagues' study, suggesting that ______

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

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

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

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