总体为样本，若是未知参数的无偏估计，则a=（）.

• A.<img src="http://timg.fentiku.com/javaimportItems/e01c6f3ea2cc4e7daa7f73d03fef654d/images/62ef.wmf"><br/>
• B.<img src="http://timg.fentiku.com/javaimportItems/e01c6f3ea2cc4e7daa7f73d03fef654d/images/6523.wmf"><br/>
• C.<img src="http://timg.fentiku.com/javaimportItems/e01c6f3ea2cc4e7daa7f73d03fef654d/images/6757.wmf"><br/>
• D.1<br/>

正确答案：C