下面是力的作用效果的几个实例。请把它们分成两类，并写出各类的特征：

A．头顶足球，足球弹回去

B．人坐沙发，沙发凹陷

C．人用力蹬自行车，车速度加快

D．运动员拉开弓第一类               ，特征                                ；

第二类               ，特征                                。

An analyst doesresearch about closely-held companies. With respect to valuation of closely-held companies, the most appropriate base for estimating a discount for lack of liquidity is the:（）

A. market price of a similar but actively publicly traded company.

B. Estimate of the value of equitywith all control rights.

C. mean value of the company's transaction prices recently.