18. Laura is a manager for HP. When Laura must decide whether to produce a few additional printers, she is choosing at the margin when she compares A} the total revenue from sales of printers to the total cost of producing all the printers. B) the extra revenue from selling a few additional printers to the extra costs of producing the printers. C} the extra revenue from selling a few additional printers to the average cost of producing the additional printers. D} I-IP's printers to printers from competing companies, such as Lexmark