题目

Since last year, short videos have gotten tons of attention online. Many people create and share short videos on popular video apps like Douyin and Kuaishou. You can even add music and special effects (特效) to your videos. The videos are 15 seconds to a few minutes long. They show people doing activities like cooking, exercising and singing. Teenagers especially enjoy these videos. Many junior high school students said they enjoyed watching these short videos. The videos let teenagers see funny and exciting things they don't usually see. Many teens like to share the fun things they do with 中国近现代史上，社会性质发生过三次变化。请指出有哪三次变化？每次变化的时间和标志分别是什么？________________________________________________________________________________________________________________________________________________________________