A non-trivial question! Not to mention you also left out a third idea: should A mention B or C?
There are many approaches to personal knowledge management. I think a “true” Zettelkasten approach would imply that the mention structure only follows A → B → C.
But many others on here and elsewhere advocate for “living notes.” In this view, you’re trying to grow a set of thoughts over time. So the A → B → C chain will eventually split out and grow in new directions. You might also add D → A and D → C.
Andy Matuschak’s notes (linked again here) are a great place to start thinking about the latter view. Sonkë Ahrens’ Take Smart Notes is a good place to look for more info about the former view.