Need help with these questions!

Hello, I'm trying to solve a test on esl.about.com, and I'm in a trouble with these questions!
1-)The test was ____ difficult she had problems finishing it on time.
A) such
B) a
C) as
D) so

I think the answer is D. However, I don't really know why I have chosen so. The test was so difficult THAT she had problems finishing it on time. I think this sentence is correct and in this quiz 'that' is omitted, which made me choose that option I guess.

And If I wanted to use such, then would the sentence have been The test was such a diffuclt test?

2-) She _______________ lunch by the time we arrived.

A-) had finished
B-) finished

To me it is finished, because both of the actions happened at the same time I suppose. But the answer is had finished. Should not it have been finished?

3-) When I stopped ________ to Mary, she was picking some flowers in her garden.

A-) to speak
B-) speaking

I think both are acceptable. Is not it like I started doing my homework or I started to do my homework? The answer is A though.

4-) Why are your hands so dirty? - I _____________ in the garden.

A-) have been working
B-) have worked

Well, I again think both are accaptable, nevertheless I think B is the correct one because It's evident that his hands are still dirty, but we really don't know whether he is still working. Again I did choose the wrong one.

5-) Which model ________ 250 k.p.h.?

A-) goes
B-) does go

It says A which I think is wrong without any doubt.


Sorry for long post. I did not want to open a new thread for each, forwhy I think It would occupy a lot.

Thanks. :-)