Let Bo and B, denote the OLS estimator of the intercept and slope of the simple linear regression model yi = Bo + Bix; + ci, i = 1, . ..,n, where c 's are independent errors with E(ci) = 0 and Var(ci) = 02. b. Let ei = yi - (Bo + 31 ;) be the residual. Prove Exie; = 0