Exercise 6:

1, be cleaned

2, be opened

3, be made

4, be recycled

5, be answered

6, be sent

7, be signed

8, be built

Exercise 7:

1, His lessons are prepared well by him before class.

2, Your lessons must be written in ink by you.

3, The plants ought to be watered by you every day.

4, The bill must be paid by them before the first of the month.

5, The mail will be delivered by the postman tomorrow.

6, Vegetables and fruits are usually eaten by the children.

7, An art exhibition will be organized by their school next month.

8, Our homework is corrected by the teacher.

9, The newspapers are delivered to this office by them every morning.

Exercise 8:

1, C – throwing litter or garbage

2, A – cut trees

3, D – making compost

4, D – cutting many trees

5, D – To prevent us from floods