If  ${x_r} = \cos (\pi /{3^r}) - i\sin (\pi /{3^r}),$ (where  $i = \sqrt{-1}),$ then value  of $x_1.x_2.x_3......\infty ,$ is :-

  • A

    $1$

  • B

    $-1$

  • C

    $-i$

  • D

    $i$

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