6. Given the quadratic equation $a x^{2}+b x+c=0(a c \neq 0)$ has two distinct real roots $m$ and $n$, and $m<|m|$. Then, the root situation of the quadratic equation $c x^{2}+(m-n) a x-a=0$ is ( ).
(A) It has two negative roots
(B) It has one positive root
(C) It has two roots of opposite signs
(D) It has no real roots
6. A.
$\because m, n$ have opposite signs and $m0 . \\
\text { Therefore, } m n=\frac{c}{a}0 . \\
\text { The discriminant of the equation } c x^{2}+(m-n) a x-a=0 \text { is } \\
\begin{array}{l}
\Delta=a^{2}(m-n)^{2}+4 a c \\
=a^{2}\left[(m-n)^{2}+4 \cdot \frac{c}{a}\right]=a^{2}(m+n)^{2} \geqslant 0,
\end{array}
\end{array}
$

Thus, it must have two real roots.
Let these two real roots be $x_{1}, r_{2}$. 
$x_{1}-x_{2}: \frac{a}{c}(n-m)0$ indicates that $x_{1}, x_{2}$ are opposite in value.