Let $S$ be the sum of all solutions (in radians) of the equation $\sin ^{4} \theta+\cos ^{4} \theta-\sin \theta \cos \theta=0$ in $[0,4 \pi]$ Then $\frac{8 \mathrm{~S}}{\pi}$ is equal to ...... .

  • [JEE MAIN 2021]
  • A

    $87$

  • B

    $78$

  • C

    $56$

  • D

    $65$

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