Natural resources: are still the most important factor in determining the productivity of human or physical capital for all countries. b) are the only factor which consistently shows a positive impact on productivity for wealthy countries. c) are a less significant source of productivity growth in most countries today than in earlier times. d) can be used to explain the differences in productivity growth among countries.

C) are a less significant source of productivity growth in most countries today than in earlier times.

Explanation:

There are basically two types of resources employed in the production of goods and manufactures: inputs, sourced from natural resources, and labor - derived from human labor.

Companies determine the amount between capital and labor that can produce a good at a lower cost.

Given that the inputs are mostly finite, the development of technologies that make it possible to increase labor productivity and rationalize the consumption of inputs in the production process dictates the rate of increase in productivity. For example, if 10 years ago a plastic factory produced 1 ton using 100kg of petroleum compost and today the same factory produces 2 tonnes with the same 100kg, there has been an increase in productivity due to the introduction of new technologies - and not by the use of the input.