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!

化学反应前后可能发生改变的是（  ）

小明学习了力的知识后，对静止在水平桌面上的文具盒来进行分析，下列说法正确的是（   ）