I believe that the 21st century is one China will bee one of the most powerful economic countries.
A. which
B. that
C. when
D. where
网友回答
A. which
B. that
(答案→)C. when
D. where
解析：句意：我相信21世纪是中国将成为最强的经济强国之一。这里one代词代替的是century，one是先行词，后跟when引导的定语从句，它在从句中作时间状语。故选C。