If You Included Both Time and Entity Fixed Effects in the Regression

Multiple Choice

If you included both time and entity fixed effects in the regression model which includes a constant, then

A) one of the explanatory variables needs to be excluded to avoid perfect multicollinearity.
B) you can use the "before and after" specification even for T > 2.
C) you must exclude one of the entity binary variables and one of the time binary variables for the OLS estimator to exist.
D) the OLS estimator no longer exists.