Question

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 ______. which choice most logically completes the text? a. 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. b. guédron, tisserand, and colleagues’ study used different methods to measure the concentration of mercury in fresh water than tjerngren and colleagues’ study did. c. 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. d. disturbances linked to wastewater affect significantly more bodies of fresh water than disturbances linked to urban development do.

Explanation:

The text discusses studies on the association between dissolved organic carbon and mercury in fresh water. Some thought the association was for undisturbed waters, but Tjemgren's study on urban - disturbed waters showed similar results. Option C logically follows: it implies that wastewater - related disturbance's effect on the association isn't representative of all human disturbances (since urban - disturbed waters had the same association as undisturbed ones). Option A compares levels in different disturbed waters, not relevant. Option B talks about methods, not the association. Option D compares the number of disturbed bodies, not the association.

Answer:

C. 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.