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 undisturbed by human activity. But Stéphane Guédron, Delphine Tisserand, and colleagues did not find this correlation in an examination of freshwater bodies impacted by wastewater, leading some scientists to hypothesize that the association could be particular to undisturbed waters. However, Ida Tjerngren and colleagues carried out a study on freshwater bodies disturbed by urban development that showed similar results to the studies on undisturbed waters, suggesting that _____

the effects of wastewater on the association between levels of dissolved organic carbon and mercury should not be taken as indicative of the effects of every type of human disturbance.

Guédron, Tisserand, and colleagues' study used different methods to measure the concentration of mercury in fresh water than Tjerngren and colleagues' study did.

disturbances linked to wastewater affect significantly more bodies of fresh water than disturbances linked to urban development do.

levels of dissolved organic carbon and mercury are both much higher in bodies of fresh water impacted by wastewater than they are in bodies of fresh water disturbed by urban development.