a realtor is trying to predict the selling price of houses in greenville (in thousands of dollars) as a function of size (measured in thousands of square feet) and whether or not there is a fireplace (fp is 0 if there is no fireplace, 1 if there is a fireplace). the regression output is provided below. some of the information has been omitted. source of variation ss df ms f regression 3177.17 2 1588.6 residual 17 17.717 total 3478.36 19 how many predictors (independent variables) were used in the regression?