Which of the Following Controls Would Be the Least Effective

Which of the following controls would be the least effective in preventing paying the same invoice twice?

A) Only pay from original invoices.
B) Cancel each document in the voucher package once the check is prepared and mailed.
C) Only pay vendor invoices that have been matched and reconciled to a purchase order and a receiving report.
D) Allow only the accounts payable department to authorize payment for vendor invoices and allow only the cash disbursements department to cut and mail checks to vendors.