The following MINITAB output presents a multiple regression equation

̂y = b0 + b1x1 + b2x2 + b3x3 + b4x4.

It is desired to drop one of the explanatory variables. Which of the following is the most appropriate action?
i. Drop x1, then see whether R2 increases.
ii. Drop x2, then see whether adjusted R2 increases.
iii. Drop x4, then see whether R2 increases.
iv. Drop x1, then see whether adjusted R2 increases.

The regression equation is Y 0.742 + 3.58 X1 - 2.18 X2 - 0.818 X3 - 1.58 X4 Predictor Constant X1 X2 X3 X4 S = 17.718 Analysis of Variance Coef StDev T P 23.310 20.447 1.1400 0.263 1.2736 0.3893 3.2713 0.003 0.1954 0.3708 0.5270 0.602 -0.6788 0.2830 -2.3981 0.023 -4.5620 3.3893 -1.3460 0.188 R-Sq = 35.0% R-Sq (adj) = 26.6% Source DF SS MS F P Regression 4 523.37 130.84 4.1678 0.008 Residual Error 31 973.21 31.394 Total 35 1496.6 42.760