2. The function $f(n)=\left\{\begin{array}{ll}n-3, & n \geqslant 1000 \\ f[f(n+7)], & n<1000\end{array}\right.$ is defined on the set of positive integers. Then the value of $f(90)$ is
A. 997
B. 998
C. 999
D. 1000
2. $\mathrm{C}$ Hint: $f(1000-k)=\left\{\begin{array}{l}997, k=4 m, \\ 1000, k=4 m+1 \text {. } \\ 999, k=4 m+2, \\ 998, k=4 m+3 .\end{array}\right.$