In a series of $2n$ observation, half of them are equal to $'a'$  and remaining half observations are equal to $' -a'$. If the standard deviation of this observations is $2$ then $\left| a \right|$ equals

  • [JEE MAIN 2013]
  • A

    $2$

  • B

    $\sqrt 2 $

  • C

    $4$

  • D

    $2\sqrt 2 $

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