题目

To stop more teenagers from becoming addicted（沉迷的） to the Internet, the Ministry of Education（MOE,教育部） sent a letter on April 20， 2018 to all parents in China. It calls on them to pay more attention to their children's online activities. Nowadays more and more teenagers become addicted to the Internet. A 17-year-old boy from Guangdong Province nearly died after playing online games for 40 hours. It shows how bad things can get when one becomes addicted to the Internet.The MOE also sent a notice to primary and middle schools, asking them to manage students' Internet use. F13．如图所示是一个密闭的内外壁均光滑的薄壁球形容器，质量为m，该球形容器刚好能放入边长为2R的立方体容器中，在金属球壳与立方体的前方、下方、右方三个接触点间安置大小可忽略的压力传感器A、B、C，传感器可以测量球形容器与立方体之间的压力大小，其中A和C处于过球心的水平圆周面上，B和C处于过球心的竖直圆周平面上．容器内有一个质量也为m的可视为质点的小球在运动．通过一段时间的观察，发现传感器A的示数一直为零，B和C的示数随时间周期性变化，若B的最大示数为4mg，则C的最大示数为（　　）A．$\frac{2}{3}$mgB．$\frac{3}{2}$mgC．mgD．$\sqrt{2}$mg