The sum of the series $1 + 3x + 6x^2 + 10x^3 + \dots \infty$ is

  • A
    $\frac{1}{(1 - x)^2}$
  • B
    $\frac{1}{1 - x}$
  • C
    $\frac{1}{(1 + x)^2}$
  • D
    $\frac{1}{(1 - x)^3}$

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