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If you are choosing your own topic for your assignment, consider the following questions:
What is the course about?
What are your main interests?
How do your interests intersect with the subject of the course?
Are there any restrictions to the topic you choose?
Pick a topic that you are genuinely interested in.
Narrow your topic as much as possible. For example, "immigration" is a huge area of research; instead, you might try something like "immigration of children from Mexico to the United States between 2015 and 2020."
While you are conducting research, you may want to modify or narrow your topic. Most of the time this can lead to a better thesis and paper. However, you should consult your assignment prompt or with your instructor if you are uncertain about whether it is okay.