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B is corrent because when there is a shipping document, this suggests that a sale occurred in the sense that goods have been shipped to a customer; accordingly an invoice should be prepared and recorded. A situation in which there is a shipping document and no invoice will ordinarily be one in which a sale has not been recorded. 
A is incorrect because each receivable may not be subsequently collected and because it address the valuation of receivables more directly than their completeness.
C is incorrect because comparing invoices to purchase orders addresses the existence of recorded sales rather than completeness.
D is incorrect because credit memos will relate to credit issues on sales as contrasted to completeness of recording of sales.