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 pattern is particular to North America. However, Yao Luo and colleagues reported dissolved organic carbon and mercury rising or falling together in a study conducted in China, and several other studies from outside North America have yielded similar results, suggesting that ______

environmental characteristics unique to Finland likely result in dissolved organic carbon and mercury levels in bodies of fresh water both being significantly higher there than elsewhere.

the bodies of water examined in Porvari and Verta's study may be more similar to the bodies of water examined in studies conducted in North America than to those examined in Luo and colleagues' study.

there may be circumstances specific to the bodies of water examined in Porvari and Verta's study that disrupted a general positive association between dissolved organic carbon and mercury levels.

bodies of fresh water outside North America are more likely to show a negative association between dissolved organic carbon and mercury levels than a positive association.