题目选项分析

Power Plant Spokesperson: "The water quality of the Phage River as it passes through Praseopolis is below federal standards. The citizens of Praseopolis often blame the poor water quality on our coal-burning power plant, which is upstream from Praseopolis. 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?

【选项】It is an explanation that the argument concludes is correct.