Read the passage given below and answer the questions based on your understanding of the passage:

When you travel around the world, you see tremendous variation in the standard of living. The average income in a rich country, such as the United States, Japan, or Germany, is more than ten times the average income in a poor country, such as India, Indonesia, or Nigeria. These large differences in income are reflected in large differences in the quality of life. People in richer countries have better nutrition, safer housing, better healthcare, and longer life expectancy as well as more automobiles, more telephones, and more televisions. Even within a country, there are large changes in the standard of living over time. In the United States over the past century, average income as measured by real GDP (gross domestic product) per person has grown by about 2 percent per year. Although 2 percent might seem small, this rate of growth implies that average income doubles every 35 years. Because of this growth, most Americans enjoy much greater economic prosperity than did their parents, grandparents, and great grandparents.

Growth rates vary substantially from country to country. In recent history, some East Asian countries, such as Singapore, South Korea, and Taiwan, have experienced economic growth of about 7 percent per year; at this rate, average income doubles every 10 years. Over the past two decades, China has enjoyed an even higher rate of growth-about 12 percent per year, according to some estimates. A country experiencing such rapid growth can, in one generation, go from being among the poorest in the world to being among the richest. By contrast, in some nations in sub-Saharan Africa, average income has been stagnant for many years. Zimbabwe has had one of the worst growth experiences: From 1991 to 2011, income per person fell by a total of 38 percent.

What explains these diverse experiences? How can rich countries maintain their high standard of living? What policies should poor countries pursue to promote more rapid growth and join the developed world? These are among the most important questions in macroeconomics. As the Nobel-Prize-winning economist Robert Lucas put it, "The consequences for human welfare in questions like these are simply staggering: Once one starts to think about them, it is hard to think about anything else."