计算 $n$ 阶行列式 $D_n=\left|\begin{array}{cccccc}a & 0 & 0 & \cdots & 0 & a \\ a & a & 0 & \cdots & 0 & 0 \\ 0 & a & a & \cdots & 0 & 0 \\ & \cdots & & \cdots & & \\ 0 & 0 & 0 & 0 & a & a\end{array}\right|$
$\text{A.}$
$\text{B.}$
$\text{C.}$
$\text{D.}$
$\text{E.}$
$\text{F.}$