问题如下：
Assuming the height of a tree in a forest follows a normal distribution, there are more than 10,000 trees. The sample size of test is 200, the 95% confidence interval of the sample mean of the height is 11 to 35 meters based on a z-statistic, the standard error of mean height is closest to:
选项：
A.1.96.
B.3.58.
C.6.12.
D.10.50.
解释：
C is correct.
考点：The central limit theorem (CLT)
解析：95%的置信区间是11到 35，z值是1.96.
平均数为0.5*（11+35）=23.所以置信区间是23 ± 1.96sx, sx是标准差。23+1.96sx等于35，sx等于6.12米
查看之前的问题，发现回答说：置信区间是通过标准误 计算出来的。
请教，如何判断置信区间是通过标准误计算还是通过标准差计算。
如果不能区分的话，对于做题而言，非常有难度。
