During a preliminary survey, an auditor found that several accounts payable vouchers for major suppliers required adjustments for duplicate payment of prior invoices. This would indicate:

(a) A need for additional testing to determine related controls and the current exposure to duplicate payments made to suppliers.

(b) The possibility of unrecorded liabilities for the amount of the overpayments.

(c) Insufficient controls in the receiving area to ensure timely notice to the accounts payable area that goods have been received and inspected.

(d) The existence of a sophisticated accounts payable system that correlates overpayments to open invoices and therefore requires no further audit concern.