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 perform which of the following?

a. Select a sample of recorded sales 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 sales 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 customer purchase orders to determine whether a valid customer actually exists.