If $f(x) = \begin{cases} x, & \text{if } x \text{ is irrational} \\ 0, & \text{if } x \text{ is rational} \end{cases}$,then $f$ is

  • A
    continuous everywhere
  • B
    discontinuous everywhere
  • C
    continuous only at $x=0$
  • D
    continuous at all rational numbers

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