The $S.D$. of the first $n$ natural numbers is

  • A

    $\frac{{n + 1}}{2}$

  • B

    $\sqrt {\frac{{n(n + 1)}}{2}} $

  • C

    $\sqrt {\frac{{{n^2} - 1}}{{12}}} $

  • D

    None of these

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