21. If we expect _____much cleaner world，we should attract ________world’s attention to protecting the world.
A．a；a B．a；/ C．a；the D．the；/
22. Mary never does any reading in the evening, _________.
A. so does John B. John does too C. John doesn’t too D. nor does John
23. Is this question worth _______ again?
A. discussing B. being discussed C. to discuss D. to be discussed
