如图所示，在地面上的物体A、B在水平力F作用下处于静止状态，现对A、B进行受力分析，以下关于A、B受力的情况中说法正确的是（   ）

阅读理解

    One day while driving from my home to work. I noticed a car in front of me had a wobbly(不稳定的) wheel. It was clear that the wheel had been changed recently and had not been fixed correctly. On a slow road this would not show much of a problem but I knew a highway was coming soon. There the traffic would move much faster and the driver would be in danger as the wheel could come off at that speed.

    I was driving a large jeep at the time and the car in front was a small one driven by a young lady on her own. So as I tried to flash(闪) my lights and wave her to pullover,  I was worried that she wouldn't pull over on the road side when she found in her mirror a single man chasing(追) her down in a jeep and waving for her to pull over.

    Thankfully. she trusted more in the good nature of people and pulled over where I could explain the problem. She said the wheel was changed only the night before and that she would pull into the nearest garage(修车间) where the repairman had tools to fix the wheel correctly.

    My belief is that the act of kindness was hers and not mine. How so? Well, it was she who chose to believe in the possibility that people are good over the belief that bad things would come from being waved over to the side of the road by strangers.

    I am glad I was able to wave her down in time and so a serious accident might have been avoided. But I am even happier that she believed in a stranger.

生物体结构和功能的基本单位是（    ）