In a regression model involving 30 observations, the following estimated regression equation was obtained: Y hat 170 +34X1 - 3X2 + X3 + 0.1X4 + 3X5 For this model, SSR = 1,740, SST = 2000. The most significant variable is: Variable Std. Error X1 1.99 0.32 1.95 0.12 0.05 X2 X3 X4 X5 X3 O X1 O X2 O X5 O X4