Let $x, y, z$ be unequal $\}$ numbers, is the inequality $x^{3}+y^{3}+z^{3}>3 x y z$ 
(A) a sufficient but not necessary condition
(B) a necessary but not sufficient condition
(C) a sufficient and necessary condition
(D) neither a sufficient nor a necessary condition
4. (A).
$x^{3}+y^{3}+z^{3}-3 x y z=\frac{1}{2}(x+y+z)\left((x \cdots y)^{2}+(y-z)^{2}+(z-x)^{2}\right)$. Therefore, and only when $x, y, \approx$ are not all equal and $x+y+z>0$, the inequality holds.