I have seen several houses. Most of them are quite unsuitable.

There are too many poor people ____ do not have enough to eat in the world.

It is much easier for a foreigner to become an American citizen if he has a close relative whoever is already an American. 

The envelop which lies on the table has no stamp on it.

=>The envelop _____ on the table has no stamp on it.

Benzene, which was discovered by Faraday, became the starting point in the manufacture of many dyes, perfumes and explosives.

=>Benzene, _____ by Faraday, became the starting point in the manufacture of many dyes, perfumes and explosives.

They are looking for the man and his dog ____ have lost the way in the forest.

My uncle, _____ you met yesterday, is a famous lawyer in the town.

Tieu Vy is the most beautiful girl _____ the Vietnam Beauty contest.