Chaco Canyon, a settlement of the ancient Anasazi culture in North America, had massive buildings. It must have been a major Anasazi center. Analysis of wood samples shows that some of the timber for the buildings came from the Chuska and San Mateo mountains, 50 miles from Chaco Canyon. Only a major cultural center would have the organizational power to import timber from 50 miles away. In the argument given, the two portions in boldface play which of the following roles?
【选项】The first is one of two premises used to support the argument`s main conclusion; the second is the other of those two premises.