题干

如图所示,表面粗糙的斜面体A放置在水平面上,物块P被平行于斜面的弹簧拴着静止在斜面上,弹簧的另一端固定在挡板B上。关于P、A受力情况的分析,正确的是(     )

A:物块P一定受到斜面体A对它的摩擦力作用

B:斜面体A一定受到水平面对它的摩擦力作用

C:斜面体A对物块P的作用力一定竖直向上

D:地面对斜面体A的作用力一定竖直向上

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D

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    Bill Gates recently predicted that online learning will make place-based colleges less significant, and five years from now, students will be able to find the best lectures in the world online. I applaud Mr. Gates. But what's taking us so long?

    As early as 1997, MIT (麻省理工) decided to post videos of all university lectures online, for free, for all people. But today, how many students have you met who mastered advanced mathematics or nuclear physics from an MIT online video? Unfortunately, the answer is not many.

    The problem is the poor quality of online education websites and the experience they provide to students. Those who go to the MIT website and watch courses online are surely very smart people, but it's not like playing a video game such as World of Warcraft. Only the most ardent students, those who are highly motivated, will devote themselves to studying these boring online videos.

    The real question is why we aren't spending more to develop better online education platforms. Where is the Avatar of education? Think about this. The market for Hollywood films per year is worth around 30 billion USD. Education in the world is a trillion-dollar-a-year market, hundreds of times bigger than Hollywood movies. Yet the most expensive digital learning system ever built cost well under 100 million dollars.

    Bill Gates' prediction is going to happen. There is no doubt about it. But it will only happen when we create high level educational content and experiences that engage and excite more than has ever been possible in the real world.