In the 1990s (last century) China's economy began to take off.
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As of 2026, 上世纪 refers specifically to the 20th century. Common in news and historical writing: 上世纪X十年代 = 'the X0s of last century', i.e. 上世纪八十年代 = 'the 1980s'. This decade-naming pattern is preferred over simply '1980s' in Chinese prose.