It's Charlie's birthday today, but he isn't feeling happy. No one wishes him a happy birthday. Mother says nothing about his birthday. She just tells Charlie to play outside. At lunchtime, his mother calls, "Come in for lunch, Charlie!" Charlie slowly walks into his house. But the living room looks different. Charlie sees a big "Happy Birthday"banner(横幅). Then, he sees a birthday cake, balloons, presents, and all of his friends. "Happy birthday!" everyone shouts. Charlie’s face lights up. He is happy and surprised(吃惊). No one forgets his birthday after all!

仔细观察图．这种堆积方式是（　　）

某学生为研究乙二酸（HOOC﹣COOH）的化学性质，进行如下实验：向盛有硫酸酸化的高锰酸钾溶液的试管中滴入适量乙二酸饱和溶液，振荡，观察到试管中溶液由紫红色变为无色．由此可知，乙二酸具有（　　）