The number of solutions of the equation $1 + {\sin ^4}\,x = {\cos ^2}\,3x,x\,\in \,\left[ { - \frac{{5\pi }}{2},\frac{{5\pi }}{2}} \right]$ is

  • [JEE MAIN 2019]
  • A

    $3$

  • B

    $4$

  • C

    $5$

  • D

    $7$

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