题目

If you travel to some towns in India, you will be lucky enough to be waited upon by guides—monkeys. These monkeys are always ready to help you. When you are hungry, you only have to point at your own stomach and they will show you to a restaurant. If you are tired, you put your hands behind your head and the monkeys will take you to a hotel. Do not feel surprised, however, when they stretch their hands out in front of you after they have helped you, they just hope you can give them a little money as a tip. After you have given them the money, they will wave their hands to say good bye. Belie如图所示，质量为m的物体从半径为R的l/4圆轨道的上端由静止滑到最低点，然后又沿水平地面运动一段距离停下来，物体与圆轨道和水平面间的滑动摩擦因数均为μ，物体全程克服摩擦力做功为（　　）A．0B．mgRC．12μRmgD．2mgR