请认真阅读下面各题，从题中所给的A、B、C、D四个选项中，选出最佳选项，并在答题卡上将该项涂黑。
21. Using _______ illegal drugs can lead to health problems, and it can be _______ stain on one’s life.
A. 不填；the B. the; the C. the; a D. 不填； a
22. Everyone present at the meeting agreed to stick to the ________that everyone should be treated fairly.
A. purpose B. ambition C. principle D. level
23. _________ in the warm sunlight, I soon fell asleep on the sunny beach.
A. Bathe B. To bathe C. Bathed D. Bathing
