Let $f: X \rightarrow X$ be such that $f(f(x)) = x$ for all $x \in X$ and $X \subseteq \mathbb{R}$. Then:

  • A
    $f$ is one-to-one
  • B
    $f$ is onto
  • C
    $f$ is one-to-one but not onto
  • D
    $f$ is both one-to-one and onto

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