(Requires Appendix Material)When the Fifth Assumption in the Fixed Effects

Multiple Choice

(Requires Appendix material) When the fifth assumption in the Fixed Effects regression (cov (u_it, u_is | X_it, X_is) = 0 for t ? s) is violated, then

A) using heteroskedastic-robust standard errors is not sufficient for correct statistical inference when using OLS.
B) the OLS estimator does not exist.
C) you can use the simple homoskedasticity-only standard errors calculated in your regression package.
D) you cannot use fixed time effects in your estimation.