21. Watching Lin Yong and his friends fly through the air ______ you wonder whether the sport is too dangerous.

A. make             B. makes                   C. making                  D. made

22. The Guineness Book of World Records is not so easy _______ .

A. to remember                     B. to be remembered  

C. remembering                    D. being remember

23.  It is one more year________ we held the 29^th Olympic Games in Beijing.

A. that                    B. after                           C. whether                     D. since