题目内容

字典中每个元素都是由（）组成的。

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（）函数可以获取字典中所有的键值对。

（）关键字表示定义一个函数。

打开文件可以使用（）函数实现。

Luxury goods are products and services that are not considered essential and associated with 1. The concept of luxury has been present in various forms since the beginning of civilization. Its role was just as important in ancient western and eastern empires 2 it is in modern societies. With the clear differences between social classes in earlier civilizations, the 3 of luxury was originally limited to the elite classes.In economics, a luxury good is a good for which demand increases more than proportionally as income rises, and is a 4 to a “necessity good”, for which demand increases proportionally 5 than income. Luxury goods are often 6 with superior goods.Luxury goods are said to have high income elasticity of demand: as people become wealthier, they will buy more and more of the luxury good. This also means, however, that should there be a 7 in income its demand will drop. Income 8 of demand is not constant with respect to income, and may change sign at different levels of income. That is to say, a luxury good may become a normal good or even an 9 good at different income levels, e.g. a wealthy person stops buying increasing numbers of luxury cars for his automobile 10 to start collecting airplanes.