Computer output from a multiple regression model with three independent variables yields the following estimated regression equation: 20.9 + 1.4X1 + 4.2X2 - 0.7X3. The standard deviations of the regression coefficients are 0.3, 1.8, 3.7 and 0.5 for b0, b1, b2 and b3 respectively. Then the most important independent variable in this model is
X1
X2
X3
cannot be determined