ГДЗ граматика Англійська мова 7 клас Мітчелл

Join the sentences using who, which, that or where, as in the example. If they can be omitted, put them in brackets. 

1. Air pollution is a serious problem (which/that) everyone should care about.
2. That’s the famous writer who/that was on TV last night.
3. That’s Jason’s cousin (who/that) I met in London.
4. That’s the flat where my aunt used to live.
5. There’s the guide who/that told us about the history of the city.
6. That’s the pencil (which/that) I used to write yesterday.