by maryadkins Wed May 16, 2012 8:56 am
Good question. I agree that (B) is tempting here.
If we check out the last paragraph, there is no question that the author believes the research is incomplete. That's clear. "Progressive" is murkier. It means progress is being made. Is it?
Well, it is in the sense that the research on birds is leading to findings that could be useful to humans. But what really counts as "progressive?" Does this sole piece of evidence on birds? If we didn't have (C), we could make an argument for (B). But since (C) is there and unquestionably true, go for it.