If $\alpha, \beta$ are the roots of the equation $x^2-2x+4=0$ and for any $n \in N, \alpha^n+\beta^n=k \cos \frac{n \pi}{3}$, then $k=$

  • A
    $2^{n+1}$
  • B
    $2^n$
  • C
    $2^{n/2+1}$
  • D
    $2^{n/2}$

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