A matrix is given.
a. Determine whether the matrix is in row-echelon form.
b. Determine whether the matrix is in reduced row-echelon form.
c. Write the system of equations for which the given matrix is the augmented matrix.
$$\left[\begin{array}{llll}
1 & 0 & 0 & 1 \\
0 & 1 & 0 & 2 \\
0 & 0 & 1 & 3
\end{array}\right]$$