What is the main criterion used by the World Bank in classifying different countries?

Solution:

The main criterion used by the World Bank in classifying different countries is the per capita income or average income of a person in a country.

Limitations of this criterion:

It does not tell us about how this average income is distributed among the people in the individual countries. Two countries with the same per capita income might be very different with regard to income distribution. One might have equitable distribution of income while the other might have great disparities between the rich and the poor.