What should I say when the application asks "Why I left a job?"

Some backstory: I'm a biology major whose first job out of college was fast food (Wendy's). A family member died a few months after i left Wendy's. Ten months later, I got hired doing science research at my second and current job. Now I'm looking to work two jobs (the research job being one of them) at the same time

1) Which would be better to explain why I left my fast food job - "personal reason" (it would be reasonable to grieve after family member died) or "looking for career change to fit my college degree"? 2) if i were to keep my second job in research and look for another part time job, should i just say that?

Edited to add: If i leave fast food job off, there will be a 4 year gap (2 of the 4 years were with the fast food job)

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