In a polluted area around the Great Lakes, scientists studying bird populations have observed a drop in the number of both herring gulls and terns. Both bird species prey on the same type of fish, whose numbers are also being negatively affected by pollution. However, the tern population has seen a much sharper decrease than the herring gull population. Scientists have concluded that the herring gulls have a higher survival rate because they are able to find alternative sources of food, including garbage dumps along the shoreline.
Which of the following, if true, would provide the best piece of evidence in support of the researchers’ conclusion?
* A survey shows that bird populations in areas around the Great Lakes have shown similar decreases.
* A related study finds that fish populations in the Great Lakes are affected as much by other predators as by pollution.
* Maps of the area reveal that the largest garbage dump in the area is located 3 miles inland from the lake.
* Blood tests performed on the gulls reveal high levels of the types of fatty acids generally found only in human food.
* Analysis of water in the area where the birds nest reveals that toxin levels are actually slightly below fish tolerance levels.
Why can't C be the best answer ?
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