Which matrix is in row echelon form? a. open square brackets table row 1 2 9 row 0 0 28 end table close square brackets b. open square brackets table row 1 0 1 row 0 1 4 end table close square brackets c. open square brackets table row 1 2 9 row 3 cell negative 1 end cell cell negative 1 end cell end table close square brackets d. open square brackets table row 1 2 9 row 0 1 4 end table close square brackets