作文：
cell phone needs great quantities of expensive metals, such as gold, silver and.....The prices of these metals rise and down dramatically in recent years. We lock costs to purchase great quantities of the metals to ......., so we can lock savings that current products require and guarantee continued profits growth.
大概就是金属价格波动，所以我们存金属来保证收益
大宗购买那些现今产品需要的材料来保证未来供给和利润
阅读：
1. 经济学家认为衡量good&service是通过extent of desire which people are willing to pay。他们采用这种方法例如问人们愿意对XX项目付多少钱in hypothetical marketplace（高亮）来了解人们的需求