To determine whether piaget's stages are universal, brad and greg conduct a research project in which 20 children in the united states and 20 children in ireland complete several conservation tasks. they are conducting a study.

They are conductin a comparative study.

Explanation:

Comparative research is a methodology used to compare results accross different populations. In this case, Brad and Greg have chosen two different countries, United States and Ireland, in order to compare results and verify if Piaget's theory can be generalized and taken as universal. Comparative research allows researchers to analize variables that vary from one culture to another and that are a result of social and cultural environments that corresond to a specific time and space.

The theory by Charles Darwin about the survival of the strong and the elimination of the weak  is known as:

According to Charles Darwin, he believed that in any given ecology or habitat, there exists some animals or organisms which are stronger than others and are able to adapt better than others.

As a result of this, he opined that the strongest would always survive the perils of nature and thrive and their species would live on, while the weaker ones who cannot adapt, would be systematically wiped off

Therefore, the correct answer is Natural selection

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