Correct the following sentences

Correct the following sentences
1.This is the girl who I sat next to her at the meeting yesterday.
2.The letter I posted it to my mother over a week ago has not reached her yet.
3.A friend who help you in time of need is a real friend.
4.Water is a liquid which boiling point is 100oC.
5.Now they are living in a new house where they built last year.
6.He wanted to go to the place where he had visited ten years before.
7.The office which he works is full of all kinds of books.
8.October 1,1949 is a day when brought the Chinese people happiness of liberation.
9.They make machines most of them will be exported to other countries.
10.The strange force is gravity,with that the earth pulls all bodies.