The head teacher of the institute ________ an M.A in English so he can also teach us.
2) has done
3) have done
4) has doing
I think the problem I'm facing is 'so'. Here, 'so' is a subordinating conjunction. The rule of subordinating conjunctions says it should be grammatically equal between the main clause and the dependent clause. If I put any of the options as an answer, it will be wrong, because, "so he can also teach us" is in present form, and the options given are in either simple past or present perfect.