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.