In a two - tailed hypothesis test, Jack Olson observes a t - statistic of -1.38 based on a sample of 20 observations where the population mean is zero. If you choose a 5 percent significance level, you should:（）
A. reject the null hypothesis and conclude that the population mean is significantly different from zero.
B. fail to reject the null hypothesis that the population mean is not significantly different from zero.
C. reject the null hypothesis and conclude that the population mean is not significantly different from zero.
参考答案：B
解析：
At a 5 percent significance level, the critical t - statistic using the Student’s t-distribution table for a two - tailed test and 19 degrees of freedom ( sample size of 20 less 1 ) is ±2.093 ( with a large sample size the critical Z - statistic of 1. 960 may be used). Because the critical t - statistic of -2.093 is to the left of the calculated t - statistic of -1.38, meaning that the calculated t-statistic is not in the rejection range, we fail to reject the null hypothesis that the population mean is not significantly different from 0.