5. Given that the radius $O D$ of $\odot O$ is perpendicular to the chord $A B$, and intersects $A B$ at point $C$. Connect $A O$ and extend it to intersect $\odot O$ at point $E$. If $A B=8, C D=2$, then the area of $\triangle B C E$ is ( ).
(A) 12
(B) 15
(C) 16
(D) 18
5. A.

As shown in Figure 4, let $O C=x$.
Then $O A=O D=x+2$.
In the right triangle $\triangle O A C$, by
the Pythagorean theorem, we have
$$
\begin{array}{l}
O C^{2}+A C^{2}=O A^{2} \\
\Rightarrow x^{2}+4^{2}=(x+2)^{2} \\
\Rightarrow x=3 .
\end{array}
$$

Since $O C$ is the midline of $\triangle A B E$, we have
$$
B E=2 O C=6 .
$$

Therefore, the area of the right triangle $\triangle B C E$ is $\frac{1}{2} C B \cdot B E=12$.