Consider the panel data regression model y = Bo + XitB +aj+ Uit where (Bo, B1) are constant parameters, x; is the regressor that is time-varying, aj is the unobserved effect, and uit is the idiosyncratic error term. The fixed effects estimator is unbiased and consistent under the assumption that a. theregressar xit is strictiy exogenaus. b. theregressor xit is uncorrelated with 4. c. theregressor xit is uncorrelated with the unobserved effect aj. d. allofthe above