Suppose ( A ) is a ( 4 times 3 ) matrix and ( b ) is a vector in ( {R}⁴ ) with the property that ( Ax = b ) has a unique solution. What can you say about the reduced echelon form of ( A )?
a) It must have at least one row of zeros.
b) It must have at least one column of zeros.
c) It must have exactly three pivot positions.
d) It must have exactly four pivot positions.