In a series of $2n$ observations, half of them equal to $a$ and remaining half equal to $-a$. If the standard deviation of the observations is $2$, then $|a|$ equals

  • [AIEEE 2004]
  • A

    $\frac{{\sqrt 2 }}{n}$

  • B

    $\sqrt 2 $

  • C

    $2$

  • D

    $\frac{1}{n}$

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