Điền vào chỗ trống 1 đại từ/trạng từ quan hệ đúng

Điền vào chỗ trống 1 đại từ/trạng từ quan hệ đúng.
1. His parents did not know the reason            he chose to be an English teacher.
2. English is a West Germanic language                   was first spoken in early medieval England.
3. Ann is a student                    is very interested in learning new languages.
4. “English Grammar in use” is the book                every student should have.
5. Many students              written English is rather good fail to get fluent in speaking it.
6. If you want to speak English as a native, you should live for at least 5 years in a country             English is the first language.