21. The disappearance of dinosaurs is not necessarily caused by astronomical incidents. But ____ explanations are hard to find.
A. alternative B. aggressive C. ambiguous D. apparent
22.I can remember very few occasions he had to ask for leave because of ill health.
A. who B. which C. when D. why
23. around the Water Cube and the Bird’s Nest，the tourists from Taiwan and I wished there _____a joint team for the London Olympics.
A. Having shown；to be B. To be shown；is C. To show；were D. Having been shown；to be
24. The thief was to three years in prison after he admitted into Mrs Robinson’s house that night.
A.caught；to have broken B.caught；breaking C.sentenced；to break D.sentenced；breaking
