6 . For the equation about $x$
$$
\begin{array}{l}
\left|x^{2}+b_{1} x+c_{1}\right|+\left|x^{2}+b_{2} x+c_{2}\right| \\
=\left|x^{2}+b_{3} x+c_{3}\right|
\end{array}
$$

the maximum number of real roots is ( ).
(A) 2
(B) 4
(C) 6
(D) 8
6. (D).

Let $f_{i}(x)=x^{2}+b_{i} x+c_{i}(i=1,2,3)$, then the roots of the equation $\left|f_{1}(x)\right|+\left|f_{2}(x)\right|=\left|f_{3}(x)\right|$ are all the roots of the equation $f_{1}(x) \pm f_{2}(x)= \pm f_{3}(x)$. This combination represents four independent equations, and the coefficient of the quadratic term in each equation is not equal to zero. Therefore, the original equation has at most 8 real roots.