For a panel data regression model, the fixed effects estimator may be obtained from the OLS estimation of a dummy variable regression, where the "dummy variable" is defined for O a. each regressor in the sample. O b. each time period in the sample. O c. each individual unit in the sample. O d. a special form of IV. O e. None of the listed answers is correct. Consider a panel data regression model with a key regressor that is correlated with the time-invariant fixed term. A/An can be used to estimate the coefficient on the key regressor. O a. first difference model O b. error correction model O c. AR(1) model O d. finite distributed lag model e. There is not enough information available to provide an