为什么是21th century 而不是21st

21st Century Breakdown “21st Century”中的“st”是什么意思? 18th or 19th centuries 为什么用复数?而不是18th or 19th century. Life in the 21st century will be different from life in th 21st century是外国人编的吗 the 21st century is hte century of information t 21st century girl 中文歌词翻译~ 21st Century 在哪能买到? 21st century.21世纪.请问"21st:"怎么读? the 21st century中st怎么读 We meet the problem facing the 21st century 为什么用facing 21th century难不难啊