【简答题】Social customs and ways of behaving have changed. Things which were considered impolite many years ago are now acceptable. Just a few years ago, it was considered impolite behaviour for a man to smoke...
【简答题】橡皮是必不可缺少的文具.现在普遍使用的塑料橡皮,主要以氯乙烯基树脂为原料.氯乙烯基树脂是由氯乙烯单体(C 2 H 3 Cl)聚合而成,这种小粒子有着很好的附着性和黏着性,因而清洁效果好. (1)C 2 H 3 Cl中碳、氢、氯元素的原子个数比是C:H:Cl=______; (2)通过计算求C 2 H 3 Cl中碳元素的质量分数.