4. (20p) True, False, Uncertain with Explanation: (a) (5p) If the key explanatory variable is constant over time, we cannot use fixed effects to estimate its effect on y (the dependent variable). (b) (5p) Using fixed effects is mechanically the same as allowing a different intercept for each cross-sectional unit. (c) (5p) In the fixed-effects regression model, you should exclude one of the binary variables for the entities when an intercept is present in the equation. (d) (5p) Time fixed effects regressions are useful in dealing with omitted variables if these omitted variables are constant over time but not across entities.