3. (25 points) The article "Experimental Design Approach for the Optimization of the Separation of Enantiomers in Preparative Liquid Chromatography" (S. Lai and Z. Lin, Separation Science and Technology, 2002: 847-875) describes an experiment involving a chemical process designed to separate enantiomers. A model was fit to estimate the cycle time (y) in terms of the flow rate (x1), sample concentration (x2), and mobile-phase composition (x3). The results of a least-squares fit are presented in the following table. (The article did not provide the value of the t statistic for the constant term). Predictor Coefficient T P Constant 1.603 X -0.619 -22.289 0.000 0.086 3.084 0.018 0.306 11.011 0.000 X1 0.272 8.542 0.000 0.057 1.802 0.115 x2 0.105 3.300 0.013 X1X2 0.022 0.630 0.549 X1 X3 -0.036 -1.004 0.349 X2 X3 0.036 1.018 0.343 Of the following, which is the best next step in the analysis? 2 1 . Nothing needs to be done. This model is fine ii . Drop x], x2 and x? from the model, and then perform an F test. ill . Drop X1X2, X1X3, and x2X3 from the model, and then perform an F test. iV . Drop x1 and x7 from the model, and then perform an F test. V. Add cubic terms x1, x2, and x3 to the model to try to improve the fit