题目

It's easy to catch the flu.Just breathe.Tiny，germ－laden particles(微粒) in the air may cause half or more of all cases of the respiratory(呼吸的) infection.And that is a surprise，scientists now report.Until now，researchers thought large particles—the type that can't stay airborne long after being expelled(驱逐) by a cough or sneeze—spread most cases of influenza. Tiny particles that can float in the air for hours to days are known as aerosols.Researchers knew that germy aerosols might spread flu.They just thought such tiny particles almost never did.Recognizing this mi近年来，美国教育部把《共.产.党宣言》公布为“美国中学生的必读书目”。阅读该宣言，美国中学生能了解到的有（ ）①阶级和阶级斗争在人类社会历史发展中的巨大作用②资本主义必然灭亡、社会主义必然胜利的美好理想③全世界无产者联合起来，共同推翻资产阶级统治④巴黎公社的经验教训A．①②③ B．①③④ C．①②④ D．②③④