If $a_k$ is the coefficient of $x^k$ in the expansion of $(1+x+x^2)^n$ for $k=0, 1, 2, \ldots, 2n$,then $a_1+2a_2+3a_3+\ldots+2na_{2n}$ is equal to

  • A
    $-a_0$
  • B
    $3^n$
  • C
    $n \cdot 3^{n+1}$
  • D
    $n \cdot 3^n$

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