Which of the following files, when compared with billing records, would provide the best source of information for determining if all goods shipped are billed to customers?

Which of the following files, when compared with billing records, would provide the best source of information for determining if all goods shipped are billed to customers?
A . Pre-numbered customer invoices.
B . Accounts receivable transactions.
C . Pre-numbered shipping documents.
D . Customer purchase orders.

Answer: C