The average marks of $10$ students in a class was $60$ with a standard deviation $4$ , while the average marks of other ten students was $40$ with a standard deviation $6$ . If all the $20$ students are taken together, their standard deviation will be

  • A

    $5$

  • B

    $7.5$

  • C

    $9.8$

  • D

    $11.2$

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