So Far?

I say it's B, but a grammarian will have to give you the official reason. I can rewrite the main thought of the sentence like this: They have not arrived at any clear understanding to date because they have not excavated enough tombs.

“So far” means “up to now”. Therefore, you cannot use the past participle after it.

Exactly! So far tells you what tense is correct for the context. B is the correct answer.

It's true that "so far" means "up to now", but the past participle "excavated" is used after it quite correctly.

The present perfect tense (have + past participle) is correct with "so far", but not the past perfect tense (had + past participle).

CJ

If it is not right to use SO FAR with a PAST TENSE, what makes it justifiable for CALD (Cambridge Advanced Learner's Dictionary) writers to use it in their dictionary with PAST and PAST PERFECT, in the meaning of until then?

She gave us a brief résumé of the project so far.
He told us to disregard everything we'd learned so far and start again.

That's interesting. "So far" (i.e. "thus far") + the past perfect seems ok to me. I wonder if it's another AmE/BrE difference.

MrP

So "so far" means both "until now" and "until then", I suppose?
Hmmm. It sounded a bit strange to me in that example sentence.
I don't think it's an American thing either.
In the "resume of the project" example, I'm also a bit queasy unless the utterance is made very soon after the giving of the resume.

Three months ago she gave us a resume of the project so far.

doesn't seem right to me. It sounds impossible unless she was clairvoyant.

Well, maybe it is an American thing; maybe it's a CJ thing!

CJ

Hmm. I suppose I read it as an example of reportage-in-the-past, to add a little immediacy to the story.

I remember once I had qualms about the use of "yet" in a past context, though everyone else thought it was fine. Maybe "past-context-so far-tolerance" also varies from speaker to speaker!

MrP