21. It’s not ________ good idea to do your homework all the evening without ________ break.

A. a ; a                           B. the ; a                            C. the ; the                   D. a ; the

22. AIDS control and prevention is ________to China as well as the whole world.

A. surprising            B. challenging              C. disappointing            D. tiring

23. Humans are polluting the earth day by day, ________, of course, will cause punishment sooner or later.

A. that                            B. it                             C. which                      D. what

24. ________ the witness will come in time tomorrow is unknown to the lawyer.

A. That                    B. What                       C. Whether                  D. Where