The mean of two samples of size $200$ and $300$ were found to be $25, 10$ respectively their $S.D.$ is $3$ and $4$ respectively then variance of combined sample  of size $500$ is :-

  • A

    $64$

  • B

    $65.2$

  • C

    $67.2$

  • D

    $64.2$

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