Consider the multiple regression model y = Bo + Bix1 + B2x2 + B3x3 + u The White heteroskedasticity-robust estimator of Var(B 2) assuming a sample size n is Var (B) = Li=liu; SSR a (2). Explain what r 12 is and how we obtain it. b (2). Explain what SSR2 is and how we obtain it. c (1). Explain what u ; is and how we obtain it