Which of the following internal controls would most likely prevent payment of vendors' duplicate invoices?

a. Cancelling invoices by means of a "Paid" stamp or perforation prior to approval.

b. Prenumbering unused voucher forms and periodically checking for missing numbers.

c. Sending cancelled checks to the bank reconciliation unit.

d. Determining that vouchers are properly authorized and approved and include original receiving documentation prior to check preparation.

e. Requiring two authorized signatures for voucher checks.