When You Add State Fixed Effects to a Simple Regression

Multiple Choice

When you add state fixed effects to a simple regression model for U.S.states over a certain time period,and the regression R2 increases significantly,then it is safe to assume that

A) the included explanatory variables,other than the state fixed effects,are unimportant.
B) state fixed effects account for a large amount of the variation in the data.
C) the coefficients on the other included explanatory variables will not change.
D) time fixed effects are unimportant.