1. D. all are correct

=> Dẫn chứng: some of them are young children. Others are teenagers. many are adults

2. C. working hard on the lesson

=> Dẫn chứng: Most people must work hard to learn English

3. D. at school

=> Dẫn chứng: Young children learn English at school because it is one of their subject

4. A. it's useful for their higher studies

=> Dẫn chứng: Teenagers often learn English for their higher studies because some of their books are in English at the college...

5. B. they need it for their job

=> Dẫn chứng: many adults learn English because it's useful for their job