10. As shown in Figure 4, in quadrilateral $A B C D$, $A B=B C, \angle A B C$ $=\angle C D A=90^{\circ}, B E \perp A D$ at $E, S_{\text {quadrilateral } A B C D}=8$. Then the length of $B E$ is ( ).
(A) 2
(B) 3
(C) $\sqrt{3}$
(D) $2 \sqrt{2}$
10. (D).

Draw a perpendicular from $B$ to $CD$, intersecting the extension of $DC$ at point $F$.
It can be proven that $\triangle A B F \cong \triangle C B F$.
Then $B E^{2}=8, B E=2 \sqrt{2}$.