And neither the purchase nor the sale changes the physical quantity of the commodity available to the market.
期货也改变不了市场上可购买的货物的物理数量。
A supermarket stocks the items needed by customers when they are needed in the quantity needed, and has all of these items available for sale at any time.
超市储存消费者需要的货品，其数量就是客户想要的，而且可以在任何时间进行发售。
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