如图所示，物体的质量m=4kg，与水平地面间的动摩擦因数为μ=0.2，在倾角为37°，F=10N的恒力作用下，由静止开始加速运动，当t=5s时撤去F，（g=10m/s²，sin37°=0.6，cos37°=0.8）．求：

如图所示，电源电压保持不变时，闭合开关后，当滑动变阻器滑片P向右滑动过程中，则   （      ）

空气是一种宝贵的资源，空气中体积分数约占78%的是

从方框中选择合适的选项补全对话。

A: Where have you been, Li Hai? I called you, but you weren't in.

B: ____

A: ____

B: I found that the factory was pouring waste water into the river near it. Lots of fish have died.

A: ____

B: Do you think “Greener China” can do something?

A: ____ if they know about this thing, they will prevent it. Let's write to them at once.

B: ____ I think everything will become better.