the 21st century 表示“世紀(jì)年代”不需大寫.
(The) 21st Century 作為刊物等的名字時,就需要大寫,《21世紀(jì)報(bào)》
供參考短語：
21st Century Weekly世紀(jì)英文報(bào)
The 21st Century世紀(jì)報(bào)；世紀(jì)中學(xué)生英語報(bào)；世紀(jì)
21st Century Teens世紀(jì)學(xué)生英文報(bào)
21st Century BusinessHerald世紀(jì)經(jīng)濟(jì)報(bào)導(dǎo)
The 21st Century learning世紀(jì)的學(xué)習(xí)
21st century boy世紀(jì)少年
21st Century College English世紀(jì)大學(xué)英語
21st century skills世紀(jì)能力
21st Century Business Review世紀(jì)商業(yè)評論