下列运算正确的是（      ）

如图所示，AOB是光滑水平轨道，BC是半径为R的光滑的 $\frac{1}{4}$ 固定圆弧轨道，两轨道恰好相切于B点．质量为M的小木块静止在O点，一个质量为m的子弹以某一初速度水平向右射入小木块内，并留在其中和小木块一起运动，且恰能到达圆弧轨道的最高点C（木块和子弹均可以看成质点）．求：

    I am an English girl. My name is Mary. I'm twelve. My dad and mom are English teachers. I have a brother. His name is Tom and he is thirteen years old. We are in China now. We are in Beijing No. 4 Junior High School. Tom is in Class 4, Grade 7 and I am in Class 9, Grade 7. I have a dog. My dog looks strong. It has a big mouth and two big black ears. We have many Chinese friends here. They are good.