Today's random linguistic observation: an adverb that is extraposed in an adverbial subclause can take scope over the complementizer of that subclause. Take a look at this pair:
omdat hij waarschijnlijk slaapt because he probably sleeps 'because he's probably sleeping'
omdat hij slaapt waarschijnlijk because he sleeps probably 'probably because he's sleeping'
As is clear from the English translations, in (1) the adverb waarschijnlijk 'probably' scopes below omdat 'because', while in (2) it scopes higher. Now, that an extraposed adverb in Dutch can scope relatively high should come as no surprise, but as high as the complementizer? There's two obvious ways of interpreting this contrast: (i) waarschijnlijk can indeed adjoin as high as CP and from that position it takes scope over omdat, or (ii) omdat is not as high up in the structure as one might think—e.g. it occupies a low projection in a split CP-system—allowing an extraposed adverb to take scope over it.
Unless of course the real answer is secret option number three: you'll notice that the examples in (1) and (2) don't contain a matrix clause. What if waarschijnlijk is not so much extraposed from the adverbial clause as it is a matrix adverb? Let's take a look at a more complete example:
Ze hebben hem ontslagen they have him fired omdat hij stal waarschijnlijk. because he stole probably 'They probably fired him because he stole.'
Forget all my earlier talk about waarschijnlijk adjoining all the way up to CP or omdat being base-generated lower that the highest C-position: maybe waarschijnlijk is just a matrix adverb in examples like (2) and (3) and never has any relationship with the adverbial clause. That would immediately explain its high scope, but it wouldn't mean we're out of the woods yet. For one, the combination of because-clause and adverb seems to form a constituent, as they can be fronted together to the pre-V2-position:
Omdat hij stal waarschijnlijk because he stole probably hebben ze hem ontslagen. have they him fired 'They probably fired him because he stole.'
This would seem to bring these examples in line with a construction discussed by Sjef Barbiers in the mid nineties and which I also briefly dabbled in (not in ‘Nam of course) ten years ago in unfinished and unpublished work, whereby both an argument and an adverb precede the finite verb in a declarative main clause:
De krant gisteren meldde het voorval niet. the newspaper yesterday reported the incident not 'The newspaper didn't report the incident yesterday.'
What makes this example similar to the data discussed above is that the adverb gisteren 'yesterday' takes clausal rather than nominal scope. In Sjef's analysis the object-DP de krant 'the newspaper' is in the specifier of the adverb gisteren 'yesterday', while I tried to argue that (5) is a genuine case of V3, but what both of us agreed upon, is that the adverb originates in the clausal spine, so it looks like option C might be the right one to go for after all.