A. $\frac{\sigma^{4}}{n}$
B. $\frac{2\sigma^{4}}{n}$
C. $\frac{\sigma^{4}}{n-1}$
D. $\frac{2\sigma^{4}}{n-1}$
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设（X_(1),X_(2),...,X_(10),X_(11)）是来自于正态总体Xsim N(mu,sigma^2)的样本，bar(X)=(1)/(n)sum_