For what values is $f$ continuous?

$$f(x) = \begin{cases} 0 & \textrm{if }x\textrm{ is rational}\\ 1 & \textrm{if }x\textrm{ is irrational} \end{cases}$$

(a) All rational $x$.
(b) All irrational $x$.
(c) All $x$.
(d) For no $x$.