After the recent court rulings, commercial shark fishing and shark hunting will resume in Diamond Bay. Many citizens of the communities around the bay have hailed this move, believing that a decrease in sharks will lead to an increase in all smaller fish, including the endangered Green-Gilled Silverfish, whose only ecosystem is within Diamond Bay. Ecologists, though, disagree, pointing out that a decrease in sharks will lead to a surge in Sea Lions, which are the principal predator of the Green-Gilled Silverfish. In the argument given, the two portions in boldface play which of the following roles?
【选项】The first describes evidence that has been taken as supporting a conclusion; the second states a contrary conclusion that is the main conclusion of the argument.