Well this is a hard one to think about isn't it well i think it is 'so' because if you get a sentence and replace 'thus' with 'so' then it will work :) XD :P
"Thus," like almost all of the words Shakespeare used, means the same today as it did when he used it; any good dictionary will give you the definition of this word. It means "in this way or in this...
The word thus precedes the causal phrase (or follows the conjunction and) in a compound sentence, or comes at the start of a sentence if it refers to a cause in a preceding sentence. (Thus means "so"...