The mean and the standard deviation $(s.d.)$  of five observations are $9$ and $0,$ respectively. If one of the observations is changed such that the mean of the new set of five observations becomes $10,$  then their $s.d.$  is?

  • [JEE MAIN 2018]
  • A

    $0$

  • B

    $4$

  • C

    $2$

  • D

    $1$

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