下列矩阵中不能相似于对角矩阵的是（）
A. $\left(\begin{array}{lll}1 & 1 & a \\ 0 & 2 & 2 \\ 0 & 0 & 3\end{array}\right)$
B. $\left(\begin{array}{lll}1 & -1 & 0 \\ -1 & 1 & 0 \\ 0 & 0 & 1\end{array}\right)$
C. $\left(\begin{array}{lll}1 & 1 & a \\ 0 & 2 & 0 \\ 0 & 0 & 2\end{array}\right)$
D. $\left(\begin{array}{lll}1 & 1 & a \\ 0 & 2 & 2 \\ 0 & 0 & 2\end{array}\right)$