设 $f(x)=\left\{\begin{array}{ll}1, & |x| \leq 1 \\ 0, & |x|>1\end{array}\right.$ ,则 $f\{f[f(x)]\}$ 等于
$\text{A.}$ 0
$\text{B.}$ 1
$\text{C.}$ $\left\{\begin{array}{l}1,|x| \leq 1 \\ 0,|x|>1\end{array}\right.$
$\text{D.}$ $\begin{cases}0, & |x| \leq 1 \\ 1, & |x|>1\end{cases}$