Foresters use regression to predict the volume of timber in a tree using easily measured quantities such as diameter. Let y be the volume of timber in cubic feet and x be the diameter in feet (measures at 3 feet above ground level). One set of data gives y=-30+60x. The predicted volume for a tree of 18 inches is: A) 1050 cubic feet B) 600 cubic feet C) 105 cubic feet D) 90 cubic feet E) 60 cubic feet