17-128 Given that $a, b, c$ are the lengths of the three sides of a triangle, then the value of $a^{4}+b^{4}+c^{4}-$ $2 a^{2} b^{2}-2 b^{2} c^{2}-2 c^{2} a^{2}$ is
(A) always positive.
(B) always negative.
(C) can be positive or negative.
(D) non-negative.
(China Shanxi Province Taiyuan City Junior High School Mathematics Competition, 1991)