记测量值x=x1，x2，...xn，则其平均值x的试验标准偏差为（）。

• A.<img src="https://timg.fentiku.com/javaimportItems/4c545e5e98524f598386ecd4aaeabd31/images/0.png" /><br/>
• B.<img src="https://timg.fentiku.com/javaimportItems/4c545e5e98524f598386ecd4aaeabd31/images/4ead.png" /><br/>
• C.<img src="https://timg.fentiku.com/javaimportItems/4c545e5e98524f598386ecd4aaeabd31/images/a25d.png" /><br/>
• D.<img src="https://timg.fentiku.com/javaimportItems/4c545e5e98524f598386ecd4aaeabd31/images/fd40.png" />

正确答案：B