$\text{A.}$ 6 $\text{B.}$ $2 \sqrt{7}$ $\text{C.}$ $\sqrt{43}$ $\text{D.}$ $3 \sqrt{3}$
【答案】 B

【解析】 $\because E F //| A B$. 易证

\begin{aligned} & \therefore C E=A E=6 \\ & \therefore D E=8-6=2 \\ & O F=4-2=2{ }^B \\ & O C=\sqrt{4^2-2^2}=2 \sqrt{3} \end{aligned}
$$\therefore B C=\sqrt{O B^2+O C^2}=\sqrt{4^2+(2 \sqrt{3})^2}=2 \sqrt{7}$$