Which of the following matrices is in row echelon form?
(0110)\begin{pmatrix} 0 & 1 \\ 1 & 0 \end{pmatrix}(0110)
(1201)\begin{pmatrix} 1 & 2 \\ 0 & 1 \end{pmatrix}(1021)
(1010)\begin{pmatrix} 1 & 0 \\ 1 & 0 \end{pmatrix}(1100)
(0011)\begin{pmatrix} 0 & 0 \\ 1 & 1 \end{pmatrix}(0101)