二十世纪
word C2- 20th century
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在二十世纪六十年代，疟疾是一种非常严重的全球性疾病。
据统计，二十世纪中期以前，黄河每年带到下游的泥沙平均约有十六亿吨，含沙量在世界大河中首屈一指。
从二十世纪下半叶开始，人们在这里尝试了多种办法。
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