力的三要素是（）、（）和（）。
参考答案：力的大小、力的方向、力的作用点
Where does the cloud come from?[ ]
A. It comes from the clouds.
B. It comes from the vapour.
C. It comes from the water.
D. It comes from the water in the river.
计算：（n-2m）（-n-2m）=______．
