题目

The predictability of our death rates is something that has long puzzled social scientists. After all, there is no natural reason why 2,500 people should accidentally shoot themselves each year or why 7,000 should drown or 55,000 die in their cars. No one establishes a quota (定额) for each type of death. It just happens that they follow a consistent pattern year after year.  A few years ago a Canadian psychologist named Gerald Wilde became interested in this phenomenon. He noticed that mortality rates for violent and accidental deaths throughout the Western world have remained stran如图1所示，物体A以速度v0做平抛运动，落地时水平方向的位移和竖直方向的位移均为L，图1中的虚线是A做平抛运动的轨迹.图2中的曲线是一光滑轨道，轨道的形状与图1中的虚线相同.让物体B从轨道顶端无初速下滑，B下滑过程中没有脱离轨道.物体A、B都可以看作质点.重力加速度为g.则下列说法正确的是 A. A、B两物体落地时的速度方向相同B. A、B两物体落地时的速度大小相等C. 物体B落地时水平方向的速度大小为D. 物体B落地时重力的瞬时功率为