If $a\cos \theta + b\sin \theta = m$ and $a\sin \theta - b\cos \theta = n,$ then ${a^2} + {b^2} = $

  • A

    $m + n$

  • B

    ${m^2} - {n^2}$

  • C

    ${m^2} + {n^2}$

  • D

    None of these

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