A Grammatical Question

Hello all, I encountered this problem in my textbook. The question is as below:

Q: This story shows that it is wrong to tell lies, ______ if you do, you have to suffer.
(A)but (B)therefore (C)and that (D)so

The answer key says (C), but I think the best answer should be (D). My reason is if the answer is (C), it means "and" is used as a conjunction between two "that" noun clauses, so it doesn't need a comma after the first clause. On the other hand, if the answer is (D), "so" acts as a conjunction between two sentences, which are "this story shows that is tis wrong to tell lies" and "if you do, you have to suffer". Therefore, the comma seems suitable between the two sentences.

I was wondering if my understanding is right, so sincerely asking for help here.

Thank you.