4: If she had a visa, she could have gone to that country.
6: If I know the meaning of the word, I don't have to ...
17: Please tell me te reason why you were absent from the meeting yesterday.
18: Manchester, where my brother lives in, is the larest city in the USA.
19: That the explorers lost their cell phone prevented the rescuers...
20: Our school has a good laboratory where the students practice chemistry.