6. In $\triangle A B C$, $B D$
$$
\begin{array}{l}
: D C=m: n, A E: E B= \\
p: q, A P: P C=s: t, \\
P M / / E C, P N / / A D, M N
\end{array}
$$
$$
\begin{array}{l}
\text { intersects } A D \text { at } Q \text {. Then } M Q: Q N \\
=(\quad) .
\end{array}
$$
(A) $\frac{m p}{(p+q) n}$
(B) $\frac{s p}{(p+q) t}$
(C) $-\frac{m s}{(s+t) n}$
(D) None of the above answers is correct
6. A

Draw $M H / / A D$ intersecting $B C$ at $H$, then $M Q: Q N$ $=H D: D N$. Let the lengths of sides $B C, C A, A B$ be $a, b, c$ respectively. Then
$$
\begin{array}{l}
D N=\frac{A P}{A C} \cdot D C \\
=\frac{s}{s+t} \cdot \frac{n a}{m+n}, \\
A M=\frac{A P}{A C} \cdot A E \\
=\frac{s}{s+t} \cdot \frac{p c}{p+q} . \\
H D=\frac{A M}{A B} \cdot B D \\
=\frac{s p}{(s+t)(p+q)} \cdot \frac{m a}{m+n}, \\
\therefore \quad \frac{H D}{D N}=\frac{m p}{(p+q) n} .
\end{array}
$$