此题怎么判断t分布是单尾还是双尾？

问题如下：

For a sample of the past 30 monthly stock returns for McCreary, Inc., the mean return is 4% and the sample standard deviation is 20%. Since the population variance is unknown, the standard error of the sample is estimated to be:

The related t-table values are (ti,j denotes the (100-j)th percentile of t-distribution value with i degrees of freedom):

What is the 95% confidence interval for the mean monthly return?

选项：

A.

[-3.453%, 11.453%]

B.

[-2.201%, 10.201%]

C.

[-2.194%, 10.194%]

D.

[-3.464%, 11.464%]

解释：

Here the t-reliability factor is used since the population variance is unknown. Since there are 30 observations, the degrees of freedom are 30 – 1 = 29. The t-test is a two-tailed test. So the correct critical t-value is t29.25 =2.045, thus the 95% confidence interval for the mean return is: