grammar 有問題唔明#(點解)???huh

Consider the following sentences:
There is a great need for the type of adult education which is provided in Croydon, which has a multi-racial society. There is a great need for the type of adult education provided in Croydon, which has a multi-racial society.
Another is the use of currency ‘hedging insurance’ which is provided by the forward foreign exchange market or by the foreign exchange options market. Another is the use of currency ‘hedging insurance’ provided by the forward foreign exchange market or by the foreign exchange options market.
Postcoding saddles, a service which is provided free by the police, is considered essential. Postcoding saddles, a service provided free by the police, is considered essential.


In meaning
1 is the same as 2
3 is the same as 4
5 is the same as 6


In other words, you can remove "which is" from "which is provided", and
you can only do when you're using passive voice.

2012-03-12 16:47:01 補充：
I've found some grammar exercises for you, too :)

http://pagenttutor.com/19979366173224432722.html

2012-03-12 16:48:41 補充：
This company doesn't seem to be a tutorial centre but they're offering exercises and model essays online :S... It sounds quite interesting. Have you heard of it?