题目

Once upon a time, a library was just about the only place that someone could go to get a book or go to read for free. Then came the arrival of the Internet, and that changed everything. Before long, books were available on different sites on the web. It was only a matter of time until Kindle, Nook, and other e-readers swept the scene. Along with them came the e-book. Many believed that this would be the end of libraries. But libraries still exist today, and many of them even thrive(繁荣). So, how is it possible that they survived against the onslaught of e-books? For one thing, the t螺线管右端的管口正对着一个闭合线圈M（如图），线圈平面与螺线管中轴线垂直，以下哪些情况能使M向右侧摆动（　　）A．闭合开关S瞬间B．S原来闭合，断开的瞬间C．闭合S稳定后滑动变阻器R的触片P右移时D．闭合S稳定后滑动变阻器R的触片P左移时