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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