Which of these is least likely to determine credit worthiness? payment history, length of employment, personal health history, personal educational background

I think the answer may be Personal Education Background. I assume the question is talking about credit as in credit card. Payment history is a big part of credit, personal health history may be important because if you have a lot of debt that you are struggling to pay off it could affect your credit I think and length of employment I would say is important because you need a job to pay. I'm not 100% in this though, I'm just trying to help. 

Hope this helps! (:

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