33 -45 Let the integer part of $\sqrt{5}$ be $m$, and the decimal part be $n$, then the value of $m\left(m-\frac{1}{n}\right)^{3}$ is
(A) 128.
(B) -128.
(C) $10 \sqrt{5}$.
(D) $-10 \sqrt{5}$.
(Shaanxi Province, Xianyang City, Junior High School Mathematics Competition, 1990)
[Solution] $\because 2<\sqrt{5}<3$, then $m=2, \quad n=\sqrt{5}-2$.
$$
\begin{aligned}
\therefore m\left(m-\frac{1}{n}\right)^{3} & =2\left(2-\frac{1}{\sqrt{5}-2}\right)^{3}=2\left[2-\frac{\sqrt{5}+2}{(\sqrt{5})^{2}-2^{2}}\right]^{3} \\
& =2 \cdot(-\sqrt{5})^{3}=-10 \sqrt{5} .
\end{aligned}
$$

Therefore, the answer is $(D)$.