One of the most commonly tested groups of words in sentence completion exercises is the use of question words to introduce relative clauses, and of this group, the most common is WHICH.
These words are called RELATIVE PRONOUNS and RELATIVE ADVERBS, and include:
The people who went to the party really enjoyed themselves.
I will never forget the time when I fell into the lake.
I asked for the reason why he didn't come to the party.
He explained what he wanted very carefully.
She wanted to know where the toilet was.
I needed to know whose car was blocking my path.
That was the car which my brother sold last year.
These words can often be used with other prepositions:
This is the group to which I belong.
That was the event at which I met my best fried.
These bags, all of which belong to Joe, are extremely heavy.
This is the man from whom I borrowed that money.
These words can be used in one of two ways, defining relative clauses where no commase are use, as above, or non-defining relative clauses which are separated by commas:
My friend, who lives in France, is a dentist.
Last year, when I went to Dallas, it snowed very heavily.
This reastaurant, where I met my wife, is very expensive.
This man, whose wife is a teacher, works with my father.
That car, which I wanted to buy, is really comfortable.
In cases like these, the commas can act as clues, but it is more important to understand the context in which these words are used.
In the FCE exams, there is nearly always one example of these words, so it is an extremely important subject to practise.