Consider the multiple regression model with two regressors x1 and x2, where both variables are determinants of the dependent variable. when omitting x2 from the regression, there will be omitted variable bias for modifyingabove beta with caret 1: a. if upper x 2 is measured in percentages. b. only if upper x 2 is a dummy variable. c. if upper x 1 and upper x 2 are correlated. d. always.