Example RREF A matrix in reduced row-echelon form

The matrix $C$ is in reduced row-echelon form. \begin{align*} C&= \begin{bmatrix} 1&-3&0&6&0&0&-5&9\\ 0&0&0&0&1&0&3&-7\\ 0&0&0&0&0&1&7&3\\ 0&0&0&0&0&0&0&0\\ 0&0&0&0&0&0&0&0 \end{bmatrix} \end{align*} This matrix has two zero rows and three pivot columns. So $r=3$. Columns 1, 5, and 6 are the three pivot columns, so $D=\set{1,\,5,\,6}$ and then $F=\set{2,\,3,\,4,\,7,\,8}$.