题目内容：
请根据短文的内容，回答题。The Differences in Living Standards
The differences in living standards around the world are vast. In 1993, the average American had an income of about $25,000. In the same year, the average Mexican earned $7,000, and the average Nigerian earned $1,500. Not surprisingly, this large variation in average income is reflected in various measures of the quality of life. Changes in living standards over time are also large. In the United States, incomes have historically grown about 2 percent per year (after adjusting for changes in the cost of living). At this rate, average income doubles every 35 years. In some countries, economic growth has been even more rapid. In Japan, for instance, average income has doubled in the past 20 years, and in South Korea it has doubled in the past 10 years.
What explains these large differences in living standards among countries and over time? The answer is surprisingly simple. Almost all variation in living standards is attributable to differences in countries' productivity- that is, the amount of goods and services produced from each hour of a worker's time. In nations where workers can produce a large quantity of goods and services per unit of time, most people enjoy a high standard of living; in nations where workers are less productive, most people must endure a more meager existence. Similarly, the growth rate of a nation's productivity determines the growth rate of its average income.
The fundamental relationship between productivity and living standards is simple, but its implications are far-reaching. If productivity is the primary determinant of living standards, other explanations must be of secondary importance. For example, people might think that labor unions or minimum-wage laws contributed to the rise in living standards of American workers over the past century. Yet the real hero of American workers is their rising productivity.
The relationship between productivity and living standards also has great implications for public policy. When thinking about how any policy will affect living standards, the key question is how it will affect our ability to produce goods and services. To improve living standards, policy makers need to raise productivity by ensuring that workers are well educated, have the tools needed to produce goods and services, and have access to the best available technology.
Which of the following countries has enjoyed the fastest economic growth in history? A．Mexico.
B．The United States.
C．Japan.
D．South Korea.
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