It is a finding the argument seeks to explain. It is the main conclusion of the argument. It introduces a judgment that the argument opposes. It is an explanation that the argument concludes is correct.
Power Plant Spokesperson: "The water quality of the Phage River as it passes through Praseopolis is below federal standards. <b> The citizens of Praseopolis often blame the poor water quality on our coal-burning power plant, which is upstream from Praseopolis. </b> But Vulcan Refinery, along a tributary of the Phage River even further upstream, releases several toxins into the water. Moreover, over 99% of the toxic materials measured in the Phage River in the vicinity of Praseopolis are consistent with metal refining, but not with the by-products of coal combustion. "
In the power plant spokesperson's argument, the portion in boldface plays which of the following roles?