题目

回答问题In the past ten years, the use of social media has grown in a way that no one could have guessed.A main feature of social media is its volatility(易变).Trends(流行)come and go, disappearing almost as quickly as they appear.So, what were the main social media trends of 2018?Short video apps, such as Tik Tok and Douyin, took the world by storm.The Telegraph reported that Tik Tok was the 8th on Apple’s App Store in April.And Douyin had more than 300 million active users in June in China, CNBC said.The sense of community that users of short video apps get is another main feature.如图所示，截面为三角形的木块a上放置一铁块b，三角形木块竖直边靠在竖直且粗糙的竖直面上，现用竖直向上的作用力F，推动木块与铁块一起向上匀速运动，运动过程中铁块与木块始终保持相对静止，下面说法正确的是A．木块a与铁块b间一定存在摩擦力B．木块与竖直墙面间一定存在水平弹力C．木块与竖直墙面间一定存在摩擦力D．竖直向上的作用力F大小一定等于铁块与木块的重力之和