Answer:
c. focus on the effects of only one change at a time.
Explanation:
An economic model is a theoretical<em> (mathematical or not) </em>construct used to explain or make predictions about economic behavior, its goal is to <em>simplify complex processes</em> to understand economic issues. <em>It uses variables to study an specific change at a time.</em>
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The correct answer is the principle stage.
Lawrence Kohlberg was known as an American Psychologist. He is well known for his theory of stages of one's moral development. He set and defined three levels of moral development, each containing two stages. The Pre-conventional, Conventional and Post-Conventional Levels.
Using a set of skills and questions, scientists form experiments to answer a main problem. Having all the variables of the experiment as controlled besides the test sample itself, specific effects to answer their questions will come into the scene. Mainly, this is called the scientific method helps them to first, identify the problem, execute reaearch, attain a hypothesis, experiment, analyze, conclude, and summarize.
I think it would Be D. Lemme know if this helps