Which of the following statement is true? a. the echelon form of a matrix is unique.
b. the pivot positions in a matrix depend on whether row interchanges are used in the row reduction process.
c. reducing a matrix to echelon form is called the forward phase of the row reduction process.
d. whenever a system has free variables, the solution set contains many solutions.
e. a general solution of a system is an explicit description of all solutions of the system.