这里单尾和双尾应该怎么判断，总是容易错

NO.PZ2022062760000017 问题如下 Using the returns of the prior 12 months, analyst estimates the memonthly return of stoXYZ to -0.75% with a stanrerror of 2.70%. Using the t-table above, whiof the following is the 95% confinintervfor the mereturn? A.-6.69% an5.19% B.-6.63% an5.15% C.-5.60% an4.10% -5.56% an4.06% 中文解析区间=mean+z值*σmean=-0.75%σ=2.7%z值的查找自由度=12-1=11，双尾5%，单尾2.5%z值=2.201区间=-0.75% - 2.201*2.70% an-0.75% + 2.201*2.70%The confinintervis equto the memonthly return plus or minus the t-statistic times the stanrerror. To get the proper t-statistithe 0.025 column must usesinthis is a two-taileinterval. Sinthe mereturn is being estimateusing the sample observations, the appropriate grees of freem to use is equto the number of sample observations minus 1, whiis 11. Therefore, the proper statistic to use from the t-stribution is 2.201. The 95% confinintervis between -0.75% - 2.201*2.70% an-0.75% + 2.201*2.70%. 关于这个自由度能不能汇总讲一下？1.这个12月就是总体啊，为什么自由度要减12.还有后面章节的，自由度 n-1的，n-k-1的，k的这都是对应什么的？全混了

NO.PZ2022062760000017 问题如下 Using the returns of the prior 12 months, analyst estimates the memonthly return of stoXYZ to -0.75% with a stanrerror of 2.70%. Using the t-table above, whiof the following is the 95% confinintervfor the mereturn? A.-6.69% an5.19% B.-6.63% an5.15% C.-5.60% an4.10% -5.56% an4.06% 中文解析区间=mean+z值*σmean=-0.75%σ=2.7%z值的查找自由度=12-1=11，双尾5%，单尾2.5%z值=2.201区间=-0.75% - 2.201*2.70% an-0.75% + 2.201*2.70%The confinintervis equto the memonthly return plus or minus the t-statistic times the stanrerror. To get the proper t-statistithe 0.025 column must usesinthis is a two-taileinterval. Sinthe mereturn is being estimateusing the sample observations, the appropriate grees of freem to use is equto the number of sample observations minus 1, whiis 11. Therefore, the proper statistic to use from the t-stribution is 2.201. The 95% confinintervis between -0.75% - 2.201*2.70% an-0.75% + 2.201*2.70%. 为什么方差不用除以根号N呢？