Let a be an nx n matrix and b be an n x 1 matrix. which of the following statements is/are true? (i) ax -b has a unique solution if and only if b can be expressed as a linear combination of the columns of the matrix a (ii) a is singular if and only if a can be expressed as a product of elementary matrices. (iii) ax -0 has only the trivial solution if and only if the reduced row echelon form of a is in. (iv) ax - b has a unique solution if and only if det(a)-0. a. (ii) only b. i) only c. (i) and (iii) only d. (ii) and (iv) only . (1), (11) and () only