A:____

B: Lao She. He was called a “Great Master of Language”.

A : You're right.____

B : Four Generations in one Household.____

A : And it mainly talks about the life of Qi's family，doesn't it?

B: Yes， it does. Do you like Lao She?

A: Yes， but Cao Yu is my favorite writer.____

B: Have you watched any of his plays?

A: Yes. Last year 1 saw Sunrise in Beijing.____

如图为实验室常用的气垫导轨验证动量守恒的装置．两带有等宽遮光条的滑块A和B，质量分别为m_A、m_B，在A、B间用细线水平压住一轻弹簧，将其置于气垫导轨上，调节导轨使其能实现自由静止，这是表明____，烧断细线，滑块A、B被弹簧弹开，光电门C、D记录下两遮光条通过的时间分别为t_A和t_B，若有关系式____，则说明该实验动量守恒．

下面是小明使用天平的几点做法，其中正确的是(　　)