27.She must type the letters this afternoon.

26.You can use a dictionary in the exam.

25. We must water the trees as often as possible.

24．You must correct your mistakes in your exercises.

23．Students must clean their classroom every day.

22. ----- Why should trees _______ (plant) on the mountains?

　 ----Forests are very important to the world. The wind can ______ (stop )from _____ (blow) the earth away. The sand can ______ (stop ) from______ (move) towards the rich farmland.

　-----We ______ already_________ (plant) about eight thousand trees. Millions of trees may _______ (need) for all the mountains here, you’ll ______ (see) a “Great Green Wall” in a few years’ time.

　----That’ll be really wonderful.

21. ----Do you know how to grow cotton? 　

----Yes, more or less.

----Can cotton _______ (grow) in water? 　

-----No, It’s usually _________ (grow) on the ground.

----- Can cotton _______ (harvest) once a year?

----Yes, It can’t _______ (harvest) twice a year.

20.I _____ (see)your uncle when I _____ (walk)in the street yesterday

19.The scientists said the world's population _____ (slow)down in a few years' time．

18._____ (have)confidence is very important