从A、B、C、D四个选项中，选出可以填入空白处的最佳选项，并在答题卡上将该项涂黑。
21. ___________the new building with the old one, we find that the new one looks more beautiful.
A. Compared B. Being compared C. Comparing D. To compare
22. While __________ TV, someone knocked at the door.
A. watched B. watching C. being watching D. he was watching
23. __________the exam, he decided to spend the weekend at home going over his lessons.
A. To pass B. Passing C. Passed D. Having been passed
