Base in or Base on ?

I also have 2 questions about clause and preposition:

  1. in the question " this is the Association’s most popular event, so … early to assure your place" . Answer: A.will register B.must register C.registering D.register. I chose C.registering but the right answer is D.register. Can you explain to me because after “So” may be a Phrase with Gerund?

  2. the second question is “this novel is based … part on a true story”
    Answer: B.on I chose B.on but the right answer is I haven’t seen Phrase " base in " before. It’s so strange for me. Can you explain to me?

  1. ‘register’ here is an instruction.

  2. ‘in part’ is a phrase meaning ‘partially’.

Thanks Miss Beeesneees so much!