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.