Choice "A" is correct. Having the check signer review and cancel the voucher packet is a preventive control to ensure the same voucher is not presented and paid a second time. Additionally, because this control is implemented prior to processing (paying for) the original invoice, it functions as a preventive control of avoiding duplicate payments. Choice "b" is incorrect. This control helps ensure that payment is made for actual goods received (documented), but canceling the voucher packets to avoid duplication of payments is a preventive control. Choice "c" is incorrect. This is not a preventive control because the bank reconciliation may detect multiple payments for the same invoice after the payments already happened. Choice "d" is incorrect. Having two check signers for material invoices is not a preventive control here. An effective preventive control is canceling the voucher packets regardless of the dollar amount of the invoices.