10. As shown in Figure 3, in quadrilateral $ABCD$, $M$ and $N$ are the midpoints of $AB$ and $CD$ respectively. $AN$, $BN$, $DM$, and $CM$ divide the quadrilateral into 7 regions with areas $S_{1}$, $S_{2}$, $S_{3}$, $S_{4}$, $S_{5}$, $S_{6}$, and $S_{7}$. Therefore, the relationship that always holds is ( ).
(A) $S_{2}+S_{6}=S_{4}$
(B) $S_{1}+S_{7}=S_{4}$
(C) $S_{2}+S_{3}=S_{4}$
(D) $S_{1}+S_{6}=S_{4}$
10. B.

Let the distances from $A$, $M$, and $B$ to $DC$ be $h_{a}$, $h_{m}$, and $h_{b}$, respectively. It is easy to see that $h_{a} + h_{b} = 2 h_{m}$. Then
$$
\begin{array}{l}
S_{\triangle D N} + S_{\triangle B N C} \\
=\frac{1}{2}\left(h_{a} + h_{b}\right) \cdot \frac{1}{2} D C = \frac{1}{2} h_{m} \cdot D C \\
= S_{\triangle A W C} .
\end{array}
$$