With regard to sales and sales returns cut-off procedures, which of the following statements is incorrect?

Select one:

a. For sales cut-off, the auditor tests a sample of sales invoices and their related shipping documents for a few days just prior to, and subsequent to, the end of the accounting period.

b. Errors related to sales cut-off can be unintentional due to delays in recognizing the shipment of goods or the recognition of the sales.

c. Immaterial sales returns can be recorded in the accounting period in which they occur, under the assumption of approximately equal, offsetting amounts at the beginning and end of each accounting periods.

d. GAAP requires the auditor to vouch the amounts of all sales returns to the original sales.