Which choice most logically completes the text?

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In a 2012 study in the United States, Michael E. Berndt and Travis K. Bavin found a positive association between levels of dissolved organic carbon and mercury in bodies of fresh water. Many other studies have yielded similar results, suggesting to some scientists that this association is true for all bodies of fresh water. But much of that research has been conducted at broadly similar sites in North America, and when Seam Noh and colleagues examined bodies of fresh water in South Korea, they found a negative association between dissolved organic carbon levels and mercury levels. If similar findings emerge from other locations outside North America, that could suggest that ____

the mercury levels reported in Noh and colleagues' study were much higher than those reported in the study by Brebdt and Bavin even though the dissolved organic carbon levels reported in the two studies were approximately the same.

Berndt and Bavin may have inadvertently measured a different characteristic of bodies of fresh water than their levels of dissolved organic carbon and mercury.

the relationship between dissolved organic carbon and mercury reported by Berndt and Bavin reflects conditions that are characteristic of certain kinds of ecosystems in North America rather than universal conditions.

most of the studies conducted in North America have measured dissolved organic carbon and mercury levels at a higher level of precision than was the case in Noh and colleagues' study.

the mercury levels reported in Noh and colleagues' study were much higher than those reported in the study by Berndt and Bavin even though the dissolved organic carbon levels reported in the two studies were approximately the same.