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. When she was doing the cleaning yesterday, Mum found the ring that she _____ years ago.
A. loses B. has lost
C. lost D. had lost
22. It was on ________ Saturday in _______ July of 2009 that the famous scholar of Chinese traditional culture, Ji Xianlin, passed away.
A. /; / B. /; the
C. a; the D. a; a
23. After decades of effort, the city is still trying to _____ how to ban smoking in public places.
A. look through B. get through
C. figure out D. take out
