$1 \cdot 63$ Let $a, b \neq 0$, the expression $\sqrt{-a^{3}} \sqrt{(-b)^{4}}$ is meaningful. Then the expression equals
(A) $-a b \sqrt{a}$.
(B) $a b \sqrt{-a}$.
(C) $a b \sqrt{a}$.
(D) $-a|b| \sqrt{-a}$.
(2nd "Five Sheep Cup" Junior High School Mathematics Competition, 1990)
[Solution] Original expression $=\sqrt{-a^{3} \sqrt{b^{4}}}=\sqrt{-a^{3} b^{2}}=|a b| \sqrt{-a}$. If $\sqrt{-a}$ is meaningful, then $a<0$.
Therefore, the original expression $=-a|b| \sqrt{-a}$, so the answer is $(D)$.