密度计是测量液体密度的工具，根据密度计浸入液体的深度就可以从刻度上读出液体的密度，你能说出其中的道理吗？

用所给单词的适当形式填空

    It was Mother's Day and I was shopping at the local supermarket with my five-year-old son,Tenyson.As we were1,we realized that only minutes earlier an2woman had fallen over at the entrance and hit her head on the ground3was with her, but there was blood everywhere and the woman was embarrassed and clearly in shock4,a lot of people stopped to help out.5we were walking towards the scene, Tenyson became very6about what had happened to the elderly couple. He7to me,“Mom, it's not much fun falling over in front of8”Seeing that there was a flower stall(摊位)at the front of the supermarket, he added, “Why shouldn't we9the lady a flower? It will make her feel better.”I was10that he'd come up with this11idea. So we went over and told the flower seller12we wanted.“Just take it,”she replied.“I13take your money for such a wonderful14”

    By now medical staff had arrived, and were15the injured woman. We gave the flower to the woman's husband and I told him it was16my son. At that, the old man started crying and said,“Thank you very much.”He then turned to me,“You have a17son. Happy Mother's Day to you.”

    The man bent down and gave his wife the flower, telling her who it was from18being badly hurt,the old lady looked up at Tenyson with19in her eyes and gave him a little20

在频率分布直方图中各小长方形的面积表示（   ）

铝作为食品污染源之一，广泛存在于各种膨化食品中，成人每日摄入量应控制在4mg以下。炸油条时需加入明矾晶体作为膨松剂。已知明矾晶体的化学式为KAl(SO4)₂·12H₂O，相对分子质量为474。