求 $n$ 阶三对角线型行列式的值：
$$
D_n=\left|\begin{array}{ccccc}
2 & 1 & 0 & \cdots & 0 \\
1 & 2 & 1 & \cdots & 0 \\
0 & 1 & 2 & \cdots & 0 \\
\cdots & \cdots & \cdots & \cdots & \cdots \\
0 & 0 & 0 & 0 & 2
\end{array}\right|
$$
$\text{A.}$
$\text{B.}$
$\text{C.}$
$\text{D.}$
$\text{E.}$
$\text{F.}$