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If A is a $k\times k$ matrix,B is a $k\times l$ matrix and C is a $l\times l$ matrix prove that:

$\det{\begin{bmatrix}A&B\\O&C\end{bmatrix}}=\det(A)\det(C)$

O is the matrix that all it's elements are equal to zero.

I know some rules for calculating determinants but I don't know how to begin in this question.