如图所示，光滑斜面倾角为30°，水平面粗糙摩擦系数为μ，现将A、B两物体（可视为质点）同时由静止释放，两物体初始位置距斜面底端O的距离为L_A=L，L_B=4L．不考虑两物体在转折O处的能量损失．求：

（1）两物体滑上水平面的时间差？

（2）若摩擦系数μ＞

（3）若摩擦系数μ=

Object-oriented analysis （OOA） is a semiformal specification technique for the object-oriented paradigm. Object-oriented analysis consists of three steps. The first step is（）.It determines how the various results are computed by the product and presents this information in the form of a（）and associated scenarios. The second is（）, which determines the classes and their attributes, then determines the interrelationships and interaction among the classes. The last step is（）, which determines the actions performed by or to each class or subclass and presents this information in the form of（）.

The first step is（）.

A．use-case modeling

B．class modeling

C．dynamic modeling

D．behavioral modeling