Using the textbook example of 420 california school districts and the regression of test scores on the student-teacher ratio, you find that the standard error on theslope coefficient is 0.51 when using the heteroskedasticity-robust formula, while it is 0.48 when employing the homoskedasticity-only formula. when calculating the t -statistic, the recommended procedure is to: a. use the homoskedasticity-only formula because the t -statistic becomes larger. b.first test for homoskedasticity of the errors and then make a decision. c.use the heteroskedasticity-robust formula. d.make a decision depending on how much different the estimate of the slope is under the two procedures.