please help me with this question

i got it from my friend. sorry don't know where the source is.

Economist:  Some sociologists argue that because capitalism intrinsically involves competition, it weakens the ties between the people of a community. Although this may formerly have been ture, modern capitalism requires that there be large corporations. Thus, modern capitalism promotes, rather than weakens, communal ties.

Which one of the following an assumption on which the economist's argument depends?

(C) Corporations that compete with each other must develop some ties to each other in order to reach agreement on the rules of the competion.

(D) Having large corporations in a modern capitalist system promotes the strength of communal ties.

I choose (C) because it solves my doubts on the "competition" mentioned in the question, and as for (D), I think it looks more like a conclusion, not an assumption. I understand why (D) could be the answer, but why not (C)? ?