which of the following matrixes are in row-reduced echelon form. ?

Reduced row echelon form has four requirements:

1) The first non-zero number in each row (the leading entry) is the number 1.

2) For every subsequent row, the number 1 must be further to the right.

3) The leading entry in each row must be the only non-zero number in its column.

4) All zero rows are placed at the bottom of the matrix.

The fourth matrix is in reduced row echelon form.

The first matrix violates the 3rd requirement

The second matrix violates the 3rd and 4th requirement

The third matrix violates the 1st requirement

The fifth matrix violates the 2nd and 3rd requirement