$9 \cdot 3 \quad$ If $a=(-1989)^{-1990}, \quad b=-1989^{1990}, \quad c=-1989^{-1990}$. Then, the size relationship of $a, b, c$ is
(A) $a=b=c$.
(B) $a>b=c$.
(C) $a>b>c$.
(D) $b<c<a$.
("Zu Chongzhi Cup" Junior High School Mathematics Invitational Competition, 1990)
[Solution] First, $a=(-1989)^{-1990}=\frac{1}{(-1989)^{1990}}>0$, while $bb$, and $a>c$.
Also, $1989^{1990}>1989^{-1990}$, thus $-1989^{1990}<-1989^{-1990}$, that is,
$$
\begin{array}{c}
b<c . \\
b<c<a .
\end{array}
$$

Therefore,
the answer is $(D)$.