Which of these matrices is lower triangular?
(0000)\begin{pmatrix} 0 & 0 \\ 0 & 0 \end{pmatrix}(0000)
(1203)\begin{pmatrix} 1 & 2 \\ 0 & 3 \end{pmatrix}(1023)
(1234)\begin{pmatrix} 1 & 2 \\ 3 & 4 \end{pmatrix}(1324)
(1023)\begin{pmatrix} 1 & 0 \\ 2 & 3 \end{pmatrix}(1203)