The fact that different countries experience different standards of living is largely explained by differences in those countries' A. populations

B. productivity levels

C. location

D. none of the above; economists are puzzled by differences in standards of living around the world

B. productivity levels

Explanation:

Worker productivity is a measurement of goods and services produced by the average worker in an hour's labor. The level of productivity is the single most significant factor in determining of the standard of living in a nation, with speedier productivity growth likely to lead to a progressively higher standard of living.

Hence the productivity level is correct answer.