4.39 Let $x=\sqrt[3]{1+991 a}$, $y=\sqrt[3]{1+992 a}$, $z=\sqrt[3]{1+993 a}$. Then, when $\sqrt{-a}$ is meaningful, the size relationship of $x, y, z$ is
(A) $x \leqslant y \leqslant z$.
(B) $y \leqslant z \leqslant x$.
(C) $z \leqslant x \leqslant y$.
(D) $z \leqslant y \leqslant x$.
(China Beijing Junior High School Mathematics Competition, 1992)
[Solution] Given that $\sqrt{-a}$ is meaningful, then $a \leqslant 0$.
Therefore, $1+993 a \leqslant 1+992 a \leqslant 1+991 a$,
which means $\sqrt[3]{1+993 a} \leqslant \sqrt[3]{1+992 a} \leqslant \sqrt[3]{1+991 a}$.
Thus, the correct choice is $(D)$.