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A is corrent because the shipping documents are prepared for the items actually shipped since those are the items sold.
B is incorrect because cash receipts would not likely occur if no sales invoice is prepared.
C is incorrect because the question is addressing the failure to prepare a sales invoice and, accordingly, beginning with the sales invoices that have been prepared is not a good approach.
D is incorrect because it is less a efficient approach than is the proper answer because certain customer orders may not be accepted (e.g., when selling on credit) and because differing amounts shipped as compared to the order may make the test complex.