But scrolling down, I can not see a single comment to any of A's posts (which ended 4 hours ago?), nor A's responses.
I finally found the little arrow in the top bar that -when clocked - shows a lot of comments to one of A's posts in the thread. Not B's posts though: I assume they must have placed the comment on one of the other posts of A. But which, and how do I open that? I don't know.