Let $\mathrm{f}: \mathrm{R} \rightarrow \mathrm{R}$ be defined by $\mathrm{f}(\mathrm{x})=\left\{\begin{array}{l}1, \text { if } \mathrm{x} \text { is rational } \\ 0, \text { if } \mathrm{x} \text { is irrational }\end{array}\right.$ then $\mathrm{ff}(\mathrm{x})$ is
(a) $x$
(b) $\mathrm{f}(\mathrm{x})$
(c) 0
(d) 1