(Requires Appendix Material) When the Fifth Assumption in the Fixed $\left( \operatorname { cov } \left( u _ { i t } , u _ { i s } \mid X _ { i i } , X _ { i s } \right) = 0 \text { for } t \neq s \right)$

Multiple Choice

(Requires Appendix material) When the fifth assumption in the Fixed Effects regression $\left( \operatorname { cov } \left( u _ { i t } , u _ { i s } \mid X _ { i i } , X _ { i s } \right) = 0 \text { for } t \neq s \right)$ 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.