2. Let $a, b, c$ be non-zero real numbers that are not all equal. If
$$
\begin{array}{l}
x=(a+b+c)^{2}-9 a b, \\
y=(a+b+c)^{2}-9 b c, \\
z=(a+b+c)^{2}-9 c a,
\end{array}
$$

then $x, y \backslash z$ ( ).
(A) are all not less than 0
(B) are all not greater than 0
(C) at least one is less than 0
(D) at least one is greater than 0
2. D.

Notice that
$$
\begin{array}{l}
x+y+z=3(a+b+c)^{2}-9(a b+b c+c a) \\
=3\left(a^{2}+b^{2}+c^{2}-a b-b c-c a\right) \\
=\frac{3}{2}\left[(a-b)^{2}+(b-c)^{2}+(c-a)^{2}\right]>0 .
\end{array}
$$

Therefore, at least one of $x, y, z$ is greater than 0.