Most household appliances use electricity only when in use. Many microwave ovens, however, have built-in clocks and so use some electricity even when they are not in use. The clocks each consume about 45 kilowatt-hours per year. Therefore, households whose microwave oven has no built-in clock use 45 kilowatt-hours per year less, on average, than do comparable households whose microwave oven is otherwise similar but has a built-in clock.Which of the following is an assumption on which the argument depends?
【选项】Microwave ovens with a built-in clock do not generally cost more to buy than microwave ovens without a built-in clock.