Secondary sources are interpretations or analyses of primary sources, often created by someone who was not directly involved in the events being discussed.
From the options provided, the three secondary sources are:
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Textbook chapter about economic markets - This is an analysis of the economic situation during the Great Depression, making it a secondary source.
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Book of historical fiction written about Dust Bowl refugees - This is a creative interpretation based on historical events, and thus it is also a secondary source.
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Documentary about the Great Depression - Documentaries generally analyze and interpret historical events, using primary sources as references, making them secondary sources as well.
The other options, including the letter home and the signed federal law, are primary sources as they provide first-hand accounts or direct evidence from that time period.