\[\mathcal {B}=\mathcal {D}=\left \{\begin{bmatrix}1 & 0\\0 & 0\end{bmatrix},\begin{bmatrix}0 & 1\\0 & 0\end{bmatrix}, \begin{bmatrix}0 & 0\\1 & 0\end{bmatrix}, \begin{bmatrix}0 & 0\\0 & 1\end{bmatrix}\right \} \]
\[\begin{bmatrix}\answer {1} & \answer {0} & \answer {0} & \answer {0}\\\answer {0} & \answer {0} & \answer {1} & \answer {0}\\\answer {0} & \answer {1} & \answer {0} & \answer {0}\\\answer {0} & \answer {0} & \answer {0} & \answer {1}\end{bmatrix}\]