If $f(x) = \frac{{\alpha \,x}}{{x + 1}},\;x \ne - 1$. Then, for what value of $\alpha $ is $f(f(x)) = x$

  • [IIT 2001]
  • A

    $\sqrt 2 $

  • B

    $ - \sqrt 2 $

  • C

    $1$

  • D

    $-1$

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