Thanks for your detailed and clear explanation, Avangi.
DougLewis, thank you too for your reply. You raised the question of when to use "what" and "which" - words that are for the most part interchangeable, but not always. The kind of words non-natives love. ;) Now we're at it, I was wondering if you could have a look at the following sentences:
What club are you in?
Which club are you in?
The clubs spoken of refer to school clubs. According to google "what" is the more common one of the two, but which one would you prefer?