题目

Alice was         the letter from Tom, but it didn't reach her for some reason. A. expecting      B. waiting       C. waiting for     D. expected   答案：A现有硫酸铜和铜的混合物18g，加足量水搅拌后过滤，在滤液中加入一定量的铁粉恰好完全反应，得到不溶物6.4g．求原混合物中铜元素的质量分数．