Determinant of block matrix with $0$ matrix in lower right

Question

Let $A$ be a $k\times k$ matrix and let $B$ be an $n\times n$ matrix. Let $C$ be $k\times n$. (All with entries in some field $K$). Suppose $D=\begin{pmatrix}C & A \\ B & 0\end{pmatrix}$. I want to find the determinant of $D$. I assume the way to go about this is using the Leibniz formula but I'm not sure where to start with that. What do I need to do?