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This statement is correct. Since the cost of processing invoices is strictly variable, the cost per unit to process customer invoices will remain the same as additional invoices are processed.
This statement is correct. Because the cost of processing invoices is strictly variable, the cost to process each invoice will be exactly the same. As a result, the incremental cost of processing one more customer invoice will be equal to the variable cost of processing one invoice. The average cost per unit for processing a customer invoice will also be equal to the variable cost of processing one invoice. Thus, the incremental cost of processing one more customer invoice will be equal to the incremental cost of processing one more customer invoice, and both will be equal to the variable cost to process one invoice.
This statement is correct. Variable costs increase in total as the volume of activity increases.
This statement is not correct. Since the cost of processing invoices is strictly variable, the cost per unit to process customer invoices will remain the same — not decline — as the volume of customer invoices increases.