Using "since" in interrogative sentences

I am confused about the usage of these conjunctions. Please help. My answer is 2/1, but I am not so sure.

(1) Why did you buy these books ___ you hardly read?

1. since
2. when
3. because
4. as

(2) ___ you hardly read, why did you buy these books?

1. since
2. when
3. because
4. as

I think I'm leaning towards when for both.

Since and because convey the same meaning, which doesn't really fit here:

since, because: for the reason that

As would also take on the same meaning in the given context.

when: in view of the fact that; considering that.

So, when seems to fit in perfectly.

Why did you buy these books when you hardly read?

When you hardly read, why did you buy these books?

However, as can also mean 'although' in certain cases, but I'd have to say it in this way to use it:

Bibliophobic as you are, why did you buy these books?

.. for lack of a better adjective :P