从A、B、C、D四个选项中选出可以填入空白处的最佳选项。
21. There are certain _____ when you must interrupt people who are in the middle of doing something.
A. conditions B. situations C. occasions D. environments
22. Jade is the highest award in Chinese culture and medals are the most important prize in Olympics, so medals with jade is the perfect _____ to present the Olympics in Beijing.
A. connection B. cooperation C. combination D. composition
