1,I have finished a large part of the novel written by Dickens,the rest of which (is) very difficult.
2,It was (he) who did it,but it is (her) that we are thinking of.
3,Three-fourths of the surface of the earth (is) covered with water.
4,This is the third time whis week he has had to study late,(isn’t it)?
5,She is in a poor (state) of health,which worries her mother much.
请解释一下为什么要这样填扩号里的词~