Assuming that ABC is exogenous, you estimate the following two specifications (numbers in parenthesis are heteroskedasticity- and autocorrelation-consistent standard errors): , = 26.08 +0.178 ABC+0.232 ABCt-1; R2=0.667, SER = 7.00 (8.83) (0.120) (0.135) , =26.08+0.178 A4BC+0.411 x ABC+-1; R2=0.667, SER=7.00 (8.17) (0.120) (0.089) a) What is the difference between the two specifications? What is the advantage of estimating the second equation, if any? b) Assuming that the errors follow an AR(1) process, you also estimate the following alternative: = -4.61+0.300 ABCt 0.070 ABC-1-0.054 ABCt-2;+0.776 Y,_ (5.84) (0.083) (0.099) (0.083) (0.057) R2=0.868, SER=4.45 How is this specification related to the previous ones? What implicit assumptions did you have to make to allow for desirable properties of the OLS estimator?