If the variance of the following frequency distribution is $50$ then $x$ is equal to:

Class $10-20$ $20-30$ $30-40$
Frequency $2$ $x$ $2$

  • [JEE MAIN 2020]
  • A

    $4$

  • B

    $-2$

  • C

    $-4$

  • D

    $2$

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