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