Which of the Following Is a Substantive Test That an Auditor

Which of the following is a substantive test that an auditor most likely would perform to verify the existence and valuation of recorded accounts payable?

A) Investigating the open purchase order file to ascertain that prenumbered purchase orders are used and accounted for.
B) Receiving the client's unopened mail for a reasonable period of time after year-end to search for unrecorded vendor's invoices.
C) Vouching selected entries in the accounts payable subsidiary ledger to purchase orders and receiving reports.
D) Confirming accounts payable balances with known vendors and suppliers who have zero balances at year-end.