If $\frac{1}{x^4+x^2+1}=\frac{Ax+B}{x^2+x+1}+\frac{Cx+D}{x^2-x+1}$,then $\cos^{-1}(A+B+C+D)=$

  • A
    $\frac{\pi}{2}$
  • B
    $0$
  • C
    $\frac{\pi}{6}$
  • D
    $\frac{\pi}{3}$

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