Based on the information provided within the question when doing research all of the answers provided are correct. Any field doing research should follow the scientific method, but Economists cannot use the same experiments as other fields because they have to use the data that the world happens to give them and do not have the ability to make control groups and test different variables on the economy like experiments in the field of Physics and Chemistry can.
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D. can result in a negative value for the coefficient of the included variable, even though the coefficient will have a significant positive effect on Y if the omitted variable were included
