$n \times n$ matrix Identity Matrix?

Can anyone explain this conceptual problem?

If an $n \times n$ matrix is in row reduced echelon form, explain why it is either the identity matrix or else has a row of zeroes?

Thanks

• The choice of a good answer here depends a lot on what you already know; there are a lot of options, but some require prior knowledge that you might not have. Do you already know that a linear system $Ax=b$ has either 0,1, or infinitely many solutions? Each of these cases can be "read off" from the echelon form (or reduced echelon form) of the augmented matrix for the system. – Ian May 17 '15 at 19:52
• Do you have an example of a matrix that is in reduced row echelon form but isn't the identity matrix? Why can't you get it in the form of the identity matrix? – Maths student May 17 '15 at 19:52