Given $(1 - 2x + 5x^2 - 10x^3) (1 + x)^n = 1 + a_1x + a_2x^2 + ....$ and that $a_1^2\,= 2a_2$ then the value of $n$ is

  • A

    $6$

  • B

    $2$

  • C

    $5$

  • D

    $3$

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