The auditor is concerned that fictitious sales have been recorded. The best audit procedure to identify the existence of the fictitious sales would be to

a. Select a sample of recorded invoices and trace to shipping documents (bills of lading and packing slips) to verify shipment of goods.

b. Select a random sample of shipping documents (bills of lading) and trace to the invoice to determine whether the invoice was properly recorded.

c. Select a sample of customer purchase orders and trace through to the generation of a sales invoice.

d. Select a sample of invoices and trace to a customer purchase order to determine whether a valid customer actually exists.