6. Determine which of the following ARMA processes are causal and which of them are invertible. (In each case {Zt} denotes white noise). a. Xt +0.2X-1 -0.48Xt-2 = Zt; b. X+1.9X-1 +0.88X-2 = Zt+ 0.2Zt-1+0.7Zt-2; c. X+0.6X-1 = Zt+1.2Zt-1; d. X+1.8X-1 +0.81Xt-2 = Zt; e. X+1.6X-1 = Zt - 0.4Z-1 +0.04Z1-2-