设x<sub>1</sub>,x<sub>2</sub>,…,x<sub>n</sub>为来自总体N(μ,σ<sup>2</sup>)的样本,μ,σ<sup>2</sup>是未知参数,则下列样本函数为统计量的是()<br /> <img src="https://nimg.ppkao.com/2019-04/wuacheng/201904121658469949.jpg?sign=ad3dc7cb271a1d7dfe39acdb33ae5073&t=62d72129" />
A、A
B、B
C、C
D、D