6. The areas of two right trapezoids are $S_{1}$ and $S_{2}$, respectively. Their oblique sides are equal, and their acute angles are complementary. The bases of one trapezoid are $4$ and $8$, and the bases of the other trapezoid are $5$ and $13$. Then $S_{1}+S_{2}$ $=(\quad)$.
(A) 96
(B) 92
(C) 88
(D) 84
6. D.

As shown in Figure 7, two right trapezoids can be combined into a large rectangle minus a small rectangle, that is,
$$
S_{1}+S_{2}=8 \times 13-4 \times 5=84 .
$$