The last 15 months of sales data are given below: Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov D 2020 13.7 14.7 14.8 13 14 13.4 13.6 14.9 13.5 14.7 15.7 219 2021 16.9 16.3 14.7 X+ represents sales in month t. Let Yt = log (Xt) and let Zt = Yt - Yt-12. Then the following model was fitted: Zt = 0.52Zt-1 +0.38Z+-2+ Et where Et is white noise. b. Using the Zt model, write down the model for Yt. Is the model for Yt stationary?