1. Consider a 2SLS system, where the causal model of interest is: yi = Bo + Biri+ ui and an instrumental variable z; is available, with E[,w;] = 0. a) Write out the "first stage" population regression model for T;. b) Write out the "reduced form" population regression model for yi. c) Show that the residual in the reduced form model is orthogonal to z; (i.e., if we call the residual v;, then E[ziv;] = 0). HINT: using the model for y and the first stage model, find an expression for the residual of the reduced form model