Bài 3. 1. How old are you?
           2. Are you fine?
           3. Is this Phong?
           4. He isn't a doctor.
           5. They aren't teachers.
Bài 4. 1. That isn't a pen.
           2. The book isn't on the table.
           3. Nam and Ba aren't very well.
           4. He isn't a doctor.
           5. The children aren't good students.