The function $f(n)$ defined on the set of natural numbers $\mathbf{N}$ is given by
$$
f(n)=\left\{\begin{array}{ll}
n-3 & (n \geqslant 1000) ; \\
f[f(n+7)] & (n<1000),
\end{array}\right.
$$

Then the value of $f(90)$ is ( ).
(A) 997;
(B) 998 ;
(C) 999;
(D) 1000 .
By special value calculation, we can guess
$$
f(n)=\left\{\begin{array}{ll}
997 & k=4 m ; \\
1000 & k=4 m+1 ; \\
999 & k=4 m+2 ; \\
998 & k=4 m+3
\end{array} \quad(m \in \mathbf{N}) .\right.
$$

This conclusion can easily be proven by mathematical induction, so $f(90)=f(4 \times 22+2)=$ 999 .