The first decade in the 21st century is the one many significant advances are made in China.
A. that
B. in which
C. which
D. where
网友回答
A. that
(答案→)B. in which
C. which
D. where
解析：考查定语从句，先行词是The first decade in the 21st century表示时间，定语从句中缺少的是时间状语，用when或in which引导定语从句，句意是：21世纪的第一个十年是中国取得很多重大进步的时间。选B。