'therefore' : " for that reason", "used to introduce the logical result of sth that has just been mentioned" 'hence' is very similar to 'therefore' and has a meaning like 'therefore' ( "that is the reason or explanation for;therefore" ) but also has a second meaning: "from this time" . In the first case I suggest you to use 'therefore' . Maybe this is just my anticipation but I have never encountered with the first meaning of 'hence' (So it is suspicious.
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