Specifying the research that needs to be performed is typically done during the stage of defining the lower-level subtopics
In the Planning stage of developing a mind map to determine the fastest way to solve Rubik's Cube, you would specify the research that needs to be performed. This stage involves identifying the key areas of inquiry and outlining the specific research questions that need to be addressed.
The Planning stage sets the foundation for the entire mind map by defining the scope and direction of the research. Specifying the research needs within this stage ensures that the subsequent stages focus on gathering relevant and actionable information.
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You are developing a mind map to determine the fastest way to solve Rubik's cube. At which stage will you specify the research that needs to be performed? 
connecting the subtopics to the central idea 
defining the lower-level subtopics 
defining the subtopics creating the image