Which of the following is a well-established observation in macroeconomics? O a. GDP per person in the richest countries in the world are about 10 times that of the poorest cour O b. In frontier economies like the United States, GDP per person grew rapidly only after the 1990s. Income per person in countries like United States and Canada has grown steadily in the last c some short-term fluctuations. O c. O d. Growth in GDP per person took off in the 1300s.