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. Quancheng Square is one of _____ largest in China, _______second to Tian’anmen Square.
A. the; the B. the: a C. /; the D. the; /
22. ______ the Atlantic Ocean crosses the equator, the trade winds cause a flow of water to the west.
A. That B. When C. Where D. Though
23. As the famous saying _______, two heads are better than one.
