计算
$$
D_n=\left|\begin{array}{ccccc}
x & a & a & \cdots & a \\
-a & x & a & \cdots & a \\
-a & -a & x & \cdots & a \\
\cdots & \cdots & \cdots & \cdots & a \\
-a & -a & -a & \cdots & x
\end{array}\right|
$$
$\text{A.}$
$\text{B.}$
$\text{C.}$
$\text{D.}$
$\text{E.}$
$\text{F.}$