Which of the following matrices is NOT in row echelon form?
(1001)\begin{pmatrix} 1 & 0 \\ 0 & 1 \end{pmatrix}(1001)
(0110)\begin{pmatrix} 0 & 1 \\ 1 & 0 \end{pmatrix}(0110)
(1501)\begin{pmatrix} 1 & 5 \\ 0 & 1 \end{pmatrix}(1051)
(0000)\begin{pmatrix} 0 & 0 \\ 0 & 0 \end{pmatrix}(0000)