The $S.D.$ of a variate $x$ is $\sigma$. The $S.D.$ of the variate $\frac{{ax + b}}{c}$ where $a, b, c$ are constant, is

  • A

    $\left( {\frac{a}{c}} \right)\,\sigma $

  • B

    $\left| {\frac{a}{c}} \right|\,\sigma $

  • C

    $\left( {\frac{{{a^2}}}{{{c^2}}}} \right)\,\sigma $

  • D

    None of these

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Subject  Mathematics Physics Chemistty
Mean $42$ $32$ $40.9$
Standard deviation $12$ $15$ $20$

Which of the three subjects shows the highest variability in marks and which shows the lowest?