补全对话 阅读下面一则对话，在空白处填入适当的句子，使补全后的对话意思完整，结构合理，上下连贯。每个空白处只能填写一个句子。[有的小题可能有多个答案，请写出你最满意的那一句。（每小题2分，共8分）

A: Hi! You look unhappy. 小题1:____________________?

B: I got very bad news from the News Report. It happened in Jiangsu Province.

A: What was it?

B:A crazy man entered a kindergarten and wounded(杀伤) 31 persons, including kids and teachers.

A: 小题2:___________________. Something must be done to keep schools safer.

B: Right. 小题3:____________________ when I grow up. Police can protect them from being hurt.

A: I think that’s a good choice. 小题4:__________________ if you want to be a policeman?

B: I am going to study harder and exercise every day to be stronger.

A: Oh, cool enough! I wish your dream come true.

B: Thank you.

如图所示，水平轨道AB与位于竖直面内半径为R=0.90 m的半圆形光滑轨道BCD相连，半圆形轨道的BD连线与AB垂直。质量为m=1.0 kg可看作质点的小滑块在恒定外力F作用下从水平轨道上的A点由静止开始向右运动，物体与水平地面间的动摩擦因数μ=0.5。到达水平轨道的末端B点时撤去外力，小滑块继续沿半圆形轨道运动，且恰好能通过轨道最高点D，滑块脱离半圆形轨道后又刚好落到A点。g取10 m/s²，求：

（1）滑块经过B点进入圆形轨道时对轨道的压力大小；

（2）滑块在AB段运动过程中恒定外力F的大小。