The correct answer is B) changes in the US economy after World War II.
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When you are doing serious research, you have to find good primary and secondary sources of information that prove to be valid. The more specific you are in what you are looking form the better results you will get. That is why it is important to use specific keywords. If not, answers and alternatives would be ambiguous and that is not what you are looking for. When you type "changes in the US economy after World War II." You are being specific and will get the proper sources.