6. Let $n \in \mathbf{Z}, f: \mathbf{Z} \rightarrow \mathbf{R}$ satisfy $f(n)=\left\{\begin{array}{ll}n-10, & n>100 \\ f(f(n+11)), & n \leqslant 100\end{array}\right.$ Among the statements above, the correct one is
A. When $n \leqslant 100$, the value of $f(n)$ remains unchanged, and is 91
B. When $n \leqslant 100$, the value of $f(n)$ remains unchanged, and is 90
C. When $n \leqslant 100$, the value of $f(n)$ changes with $n$
D. When $n<90$, the value of $f(n)$ changes with $n$
6. A Let $90 \leqslant n \leqslant 100$, then $f(n)=f^{(2)}(n+11)=f(n+11-10)=f(n+1)$, thus, $f(90)=$ $f(91)=\cdots=f(101)=91$; If $n<90$, take $m \in \mathbf{N}^{*}$, such that $90 \leqslant n+11 m \leqslant 100$, then $f(n)=f^{(2)}(n+11)$ $=\cdots=f^{(m+1)}(n+11 m)=f^{(m)}(f(n+11 m))=f^{(m)}(91)=91$.