Answer:
Choose a subject you already know very well.
Step-by-step explanation:
Choosing a subject that you already know very well is not a guideline for choosing a research subject. If you research a subject that you already know well, you will be more likely to have some bias or preconceived notions about the subject. Moreover, you will find the subject less interesting. Finally, this might diminish the impact of the research as you might not end up learning much.