Please answer the following question! No need to compare with the previous question. Thank you!

4. (UR Model; 30 pts, 10 pts each) Assume the following unit root model without a drift Y; = Yg_1 + 5;, Y0 = 0, [UR Model) {it ~ ad (0,53), and this is sometimes called as the \"stochastic trend model". (a) Using a recursive substitution, show that Y; = Z! i=1 33" (b) Derive the expression for Var (K), and conrm that it is increasing as t gets larger. Is K covariance stationary? (c) Compute the impulse response function IRFJ- = $1: with the integer j E (1, t 1). Compare this with the answer from Q3.(b) above, with some economic interpretation