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：

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      There is a park near our school. Wecan see many trees and flowers there. Some flowers are red, and some areyellow. We can see a hill behind the park too. At the foot of the hill we cansee a river. There are some boats on it. Children like to go there. It is springnow. There are many children in the park. Some are walking and singing. Some aredancing and playing games. Ann and her sister Kate are flying a kite. Tom andhis brother Sam are throwing a frisbee(飞盘).Their father and mother are sitting under the tree. They are all happy.

下列对长颈鹿的长颈形成原因的解释正确的是（  ）