I believe that the 21st century is one China will become one of the most powerful economi

问题补充：

I believe that the 21st century is one China will become one of the most powerful economic countries.

A. which

B. that

C. when

D. where

答案：

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。