下列说法中正确的是（ ）
A．物体静止时，一定不受力
B．物体运动状态改变时，一定受到力
C．物体受力时，它的运动状态一定改变
D．物体不受力时，一定处于静止状态
A、物体静止时受平衡力作用，如静止桌面上的课本，所受的支持力和重力为一对平衡力，故A错误；
B、因为力是改变物体运动状态的原因，因此物体运动状态改变，一定受力的作用，故B正确；
C、物体受平衡力时，运动状态不变，故C错误；
D、物体不受力时，物体处于静止状态或匀速直线运动状态，故D错误．
故选B．
| 完形填空。 | |||
| Look here, please! Can you see the man in black? 1 is Mr Lee, my English teacher. He teaches (教) us 2 . He is a very good teacher. 3 is he? He's forty-five. He is 4 America. He has a small yellow car. It's beautiful. It's 5 the tall building now. He goes to 6 by car every day. He leaves (离开) home at 7:00 a.m. and gets home at 5:00 p.m. Tom and Tim are his twin sons. 7 are not in our school. They are in No.1 Middle School. They are in 8 . They go to school early every morning. They are both good 9 . They are good at English. But they can't speak Chinese. Today they are not 10 school, because it is Sunday. | |||
|