For a set of 15 data points, a computer statistical package has found the multiple regression equation to be y = -23 + 20x_1+ 5x_2 + 25x_3 and has listed the t-ratio for testing the significance of each partial regression coefficient. Using the 0.05 level in testing whether b_1= 20 differs significantly from zero, the critical t values will be t = -1.960 and t= +1.960 t = -2.132 and t = +2.132 t = -2.201 and t = +2.201 t = -1.796 and t = +1.796