How can I define lines so when I click on the screen, my program knows whether I clicked on A B or C?
Ask the main part of your question in the title. This should be concise but informative.
Provide everything up front. Don't make people fish for more details in the comments. Provide background information and examples.
Be present for follow up questions. Don't ask for help and run away. Stick around to answer questions and provide more details.
Ask about the problem you're trying to solve. Don't focus too much on debugging your exact solution, as you may be going down the wrong path. Include as much information as you can about what you ultimately are trying to achieve. See more on this here: https://xyproblem.info/
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Good answer! I can't recommend a library right now, but I recommend looking into the geometric term "Point in Polygon" to further read about this task/problem.
Edit: wanted to clarify that I just wanted to give a better keyword for OP to search for a solution.