If $4{\sin ^4}x + {\cos ^4}x = 1,$ then $x =$

  • A

    $n\pi $

  • B

    $n\pi \pm {\sin ^{ - 1}}\frac{2}{5}$

  • C

    $n\pi + \frac{\pi }{6}$

  • D

    None of these

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