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 provides evidence in support of the main conclusion of the argument.