6. (adapted from II.2 5) True or false: (a) If x minimizes | |Ax - b|2, & satisfies A" Ax = Ab. (b) The least squares problem || Ax - b|, has a solution only if A"A is invertible. (c) The least squares problem ||Ax - b|, has a unique solution if A? A is invertible. (d) If x minimizes |Ax - b ,, then the residual vector r = b - Ax is in the range of A