如图所示，光滑的水平面上，小球m在拉力F作用下做匀速圆周运动，若小球到达P点时F突然发生变化，下列关于小球运动的说法正确的是（   ）

    Hello! I' m John. I usually have a busy but happy weekend. I usually do my home- work first. Then I visit my grandparents on Saturday afternoon. They are very happy to see me. On Sundays I often play football with my friends. Sometimes I go swimming or go shopping. But I can't go out this weekend, because the weather report says it's going to rain. I can't play football in the rain. I can only play the piano at home. This is my weekend. What about you?

2013年2月，全国多地、多日出现雾霾天气，致使空气中PM2.5严重超标。部分PM2.5颗粒能通过呼吸道和肺进入血液，危害人体健康。PM2.5颗粒经过鼻、咽、喉以后，进入血液之前，经过的结构依次是(           )