从A、B、C、D四个选项中，选出可以填人空白处的最佳选项，并将答案写在答题卡上。
21. In his twenties, he took part in sports and gained a great deal of __________.
A. ability B. study C. experience D. knowledge
22. If you want to _________ university, you should work hard.
A. attend B. join C. take part in D. join in
23. ___________ returning from abroad, he set about setting up a club for young climbers.
A. Upon B. At C. In D. During
